The Substitution Clause and IR35: What It Proves and What It Does Not

Why personal service sits at the heart of IR35

When HMRC or a tribunal assesses whether a contractor falls inside IR35, the starting point is the three-limb test from Ready Mixed Concrete (South East) Ltd v Minister of Pensions [1968] 2 QB 497. A contract of service (employment) requires three minimum conditions: the worker must personally perform the services, the engager must exercise a sufficient degree of control, and the overall terms must be consistent with employment rather than some other relationship. If any one of those limbs is absent in a genuine and credible way, the contract is not a contract of employment, and IR35 should not apply.

Personal service is the first limb. A genuine, unfettered right to substitute another worker to carry out the engagement breaks the personal-service requirement. It is one of the most powerful indicators available to a contractor because it directly defeats a necessary condition of the employment relationship. That is why substitution clauses matter, and why HMRC scrutinises them so closely.

This page focuses on substitution in depth. If you want to understand how the control test and mutuality of obligation interact with the three-limb structure, the overview guide to the IR35 status tests covers the full framework, and the flagship guide on what IR35 is sets out how the legislation works in practice. For the practical steps to protect an outside-IR35 position across the full range of working-practices evidence, see the guide on working outside IR35, which includes substitution alongside contract review and the broader evidence picture.

Genuine substitution: what the right actually means

A genuine substitution right has three core characteristics. First, it is unconditional in substance: the contractor may send a suitably competent replacement without needing the client's positive approval, other than a check that the substitute has the skills to do the job. Second, the contractor pays the substitute from the fee received, not the client. Third, the right is real in the sense that both parties understood and intended it when the contract was made, and would have accepted its exercise.

The requirement for the contractor to fund the substitute payment is not a technicality. If the client pays the substitute directly, there are now two separate labour arrangements, and the contractor is functioning as little more than a recruitment conduit. By contrast, when the contractor arranges and pays for the substitute from their own contract income, they are absorbing a business cost to service the client. That is the conduct of a business, not a worker. It is the financial reality that separates a genuine commercial substitution from a rebranded employment.

On the question of whether the client can vet the substitute, the answer is yes, within limits. The client may require that any substitute is appropriately qualified and capable of performing the contractual scope. That is an objective, skills-based condition. It does not undermine the right. What the client cannot do, if the clause is to carry full weight, is to insist on the named contractor in all but emergency circumstances, refuse a substitute on personal preference grounds, or reserve a general approval right that has nothing to do with competence.

What tribunals actually weigh

Case law is consistent on two points. A substitution right does not need to have been exercised to count. And a substitution clause carries weight only to the extent that it reflects the genuine commercial reality of the engagement.

In HMRC v Atholl House Productions Ltd [2022] EWCA Civ 501 and Kickabout Productions Ltd v HMRC [2022] EWCA Civ 502, the Court of Appeal confirmed that tribunals must apply a three-stage hypothetical contract test. First: identify the terms of the actual contractual arrangement. Second: ask whether, on those terms, there is sufficient mutuality of obligation and control for the contract to be one of service at all. Third: stand back and ask whether, on the whole picture, the hypothetical direct contract between the worker and the end client would be a contract of service, including asking whether the individual is in business on their own account.

Substitution evidence matters at the first and third stages. At the first stage, a genuine substitution right is part of the contractual terms to be fed into the analysis. At the third stage, the practical realities of substitution (whether it was understood to be real, whether the contractor had people to call on, whether the client accepted it was possible) contribute to the whole-picture assessment of whether the contractor was running a business or simply working as an employee behind a corporate wrapper.

The Supreme Court's decision in Professional Game Match Officials Ltd v HMRC [2024] UKSC 29 confirmed that mutuality of obligation and control are necessary but not sufficient for employment. The whole-picture assessment that follows must genuinely be carried out, and substitution can be significant evidence at that stage. But none of this changes the fundamental requirement that the substitution must be credible on the facts.

One practical implication flows directly from this case-law structure. A contractor who wants to rely on substitution at a tribunal needs more than a contract clause. They need to be able to give evidence that the right was genuine: that they understood they could exercise it, that the client understood it too, and that there was a realistic mechanism for doing so. A tribunal will typically ask whether the contractor could identify a substitute, whether they had worked with or had access to suitably qualified people, and whether the client had ever been told the right existed. Answers that reveal the contractor had not thought through the mechanics at all weaken the substitution argument even if the clause on paper is well-drafted.

There is also a timing point. A substitution right agreed at the start of an engagement but then effectively abandoned in practice as the engagement matures and the contractor becomes increasingly integrated into the client's operations is at risk of being treated as a right that existed in form but lapsed in substance. Revisiting the contract at each renewal and confirming the substitution right remains live and workable is good practice, not just legal formality.

The fettered clause: reduced weight but not worthless

A fettered clause is one that gives the contractor a right to substitute, but hedges it with conditions that allow the client to refuse for reasons that go beyond bare competence. Common examples are clauses that:

• require the client's prior consent, without any qualification as to what grounds the client may or may not rely on;
• state that the contractor will personally perform the services "except where impractical", which implies personal service is the norm and substitution the exception;
• restrict substitution to occasions when the contractor is ill or unavailable, rather than giving a free-standing commercial right;
• require that the substitute is acceptable to the client "in its absolute discretion".

A fettered clause is not fatal. Tribunals look at all the evidence, and a fettered right can still point weakly toward self-employment, particularly if other factors are strong. But it carries significantly less weight than an unfettered one, and an engagement that relies heavily on a fettered clause to argue outside-IR35 is vulnerable.

Where the fetter relates only to competence (the client can turn down a substitute who lacks the technical skills for the role), most tribunals treat that as commercially reasonable and do not regard it as truly fettering the right. The critical question is whether the client's power to refuse is about the person's ability to do the job or about the client's preference for the specific individual. The former leaves the substitution right meaningful; the latter substantially negates it.

The more the fetter resembles a de facto personal-service requirement dressed up with a nominal fallback, the closer the clause comes to carrying no weight at all. At that point it may as well not be in the contract, and the personal-service limb is effectively conceded.

If your current contract contains a fettered clause and you cannot renegotiate more favourable language, the best approach is to ensure the rest of the working-practices picture is strong: genuine financial risk, short-term engagements without automatic renewal, no integration into the client's management hierarchy, and the ability to work for other clients. Substitution is one factor; the whole picture still governs.

The sham clause: when the paperwork actively harms you

A sham clause is different in kind from a fettered one. It is a clause inserted into the contract to create the appearance of a substitution right when neither party had any genuine intention that the contractor would ever send a substitute, or that the client would ever accept one if offered.

Sham clauses arise in a number of ways. An agency may insert standard substitution language into every contractor contract because it is good IR35 hygiene on paper, regardless of whether the actual working arrangement supports it. An accountant or IR35 adviser may recommend expanding or strengthening the clause language without the underlying commercial relationship changing at all. A client may sign a contract containing a substitution right while making clear in practice that they require the specific individual and would not accept anyone else.

Tribunals are alert to this. When the clause is contradicted by the rest of the evidence, it does not merely fail to help: it can suggest that the contract was deliberately drafted to achieve a tax outcome that the actual relationship does not support. That is a pointer toward a structured arrangement to avoid employment status, which is exactly what HMRC is looking for when it opens an enquiry. A carefully drafted substitution clause sitting alongside a long exclusive engagement, fixed working hours, a dedicated desk, and a line manager telling the contractor how to do the work every day makes the overall presentation worse, not better.

The firm's consistent position is that working practices over contract wording governs. A sham clause is the most extreme version of that principle: it is wording that directly contradicts the practices. Always ensure any substitution right in your contract accurately reflects how the engagement would actually operate if you chose to exercise it.

Exercising the right in practice

The fact that a right has never been used does not mean it is not genuine. Many contractors work through a single PSC for years, rarely if ever send a substitute, and are genuinely outside IR35 because the underlying commercial arrangement supports it. But the longer the engagement runs without any substitution occurring, and the more the working practices look like employment, the more important it becomes to be able to explain why the right remained unused.

The credible answer is not "I could never have sent a substitute, the client wouldn't allow it." The credible answer is: "I was the most qualified person available, I was the one with client-specific context, and the business economics meant I chose to carry out the work personally. If I had needed to, I could have found someone and the client would have accepted them." That answer is supported by evidence: knowledge of who the substitute would have been, confirmation the client had no personal veto, and, ideally, some contemporaneous communication about the theoretical possibility.

Where substitution has actually been exercised, keep clear records: the written arrangement with the substitute, what they were paid, how the payment flowed, and any client communication confirming acceptance. These records are close to the best possible IR35 evidence on this limb, because they prove the right was not theoretical.

Paying the substitute: the financial discipline

The mechanic matters. You invoice the client at your rate, collect the fee in your PSC, pay the substitute from that fee, and retain any margin. You do not ask the client to increase the invoice or to pay the substitute separately. The economics must show that you bore the substitute's cost as a business expense.

Two things follow from this. First, you need an agreement in place with the substitute, even if informal: what they will be paid, the scope of the work, and whether they are engaged through their own limited company, on a self-employed basis, or as a PAYE worker. Second, you need to make sure you are not creating a new employment in the process: if the substitute is a director of their own PSC, they invoice you and the transaction is clean. If they are working for you as a casual worker, their own employment-status position needs consideration.

This is not a theoretical risk. HMRC has been known to look not only at the contractor's status but at the status of the substitute arrangement, particularly where the substitute is a family member or where the payment structure looks designed to move money without creating a genuine arm's-length relationship.

Substitution in the CEST tool

HMRC's Check Employment Status for Tax tool (updated 30 April 2025) includes a dedicated substitution section. It asks whether the contractor has the right to send a substitute, whether the client can turn the substitute down, and who would pay the substitute. The answers feed into CEST's personal-service analysis.

HMRC will stand behind a CEST result where the inputs are accurate, consistent with the actual working practices, given in line with the guidance, and where there is no avoidance. If your contract genuinely gives you an unfettered substitution right and the client would genuinely accept a substitute, answering CEST accurately should return a finding consistent with that reality.

Two cautions apply. First, CEST does not bind a tribunal. An "outside" result on the substitution questions is supporting evidence, not a guarantee. Second, and more importantly, answering CEST as though you have a genuine substitution right when your actual working arrangement does not support it is dangerous: the CEST result will not survive scrutiny if HMRC investigates, and the mismatch between answers and reality will itself become evidence of an attempt to contrive an outside status. CEST is only as useful as the accuracy of the inputs. Pair it with a professional contract review and an honest assessment of your working practices.

Substitution alongside the other tests

Substitution addresses personal service. The other two limbs of the Ready Mixed Concrete test are control and mutuality of obligation. A brief word on how they interact with the substitution analysis, before pointing to the dedicated pages for each.

If personal service is defeated by a genuine substitution right, there is technically no contract of employment without further analysis. In practice, however, tribunals still look at the full picture, including control and MOO, as part of the whole-picture stage. A contractor who has a strong substitution clause but who is under extensive day-to-day control (told how, when, where and in what detail to carry out the work) has not necessarily won. The substitution point reduces the personal-service weight against them, but the remaining factors are still weighed. The IR35 status tests guide covers all three limbs and the whole-picture assessment together.

On mutuality of obligation specifically: the Supreme Court confirmed in PGMOL [2024] UKSC 29 that MOO can exist within a single engagement even where there is no umbrella obligation between engagements. An absence of MOO between contracts is not a reliable outside pointer on its own. The dedicated guide on mutuality of obligation and IR35 goes into the detail of what PGMOL means in practice for contractors.

What a credible substitution position looks like in full

Pulling the threads together: a substitution clause that helps your IR35 position has six characteristics.

• It is in writing in the contract and gives an unconditional right to substitute any suitably qualified person, subject only to reasonable competence requirements.
• It specifies (or clearly implies) that the contractor pays the substitute from the contract fee.
• It does not allow the client to refuse on personal preference grounds or give the client an unqualified veto.
• It matches the actual commercial reality: the client would accept a substitute if offered, and the contractor has considered who that person might be.
• It is consistent with the rest of the contract and the working arrangements, not sitting as an isolated clause contradicted by everything else.
• There is some evidence, documentary or testimonial, that the right is genuine: a conversation recorded in email, a similar arrangement with another client, or an actual substitution that occurred and was accepted.

If your current contract falls short on any of these points, the starting place is a professional contract review that looks at the substitution language in context. The review needs to assess not just whether the words are there but whether the clause accurately reflects how the engagement operates. For a full picture of how substitution fits within the broader working-practices evidence that protects an outside-IR35 position, the guide on working outside IR35 covers the complete checklist.

Getting professional support

Substitution is one element of a robust IR35 position, and no single element is decisive. The strongest protection comes from a contract that accurately reflects genuinely self-employed working practices, a CEST run on accurate inputs, and a professional review that identifies inconsistencies before HMRC does. If you are uncertain whether your substitution clause is genuine and enforceable, or whether your current working arrangement supports the outside-IR35 status your contract claims, our contractor accountancy team covers both the contract and the working practices as part of the full specialist contractor accounting service.