IR35 and Substitution: The Most Misunderstood Test

Of all the tests used to determine IR35 status, substitution is the most powerful — and the most misunderstood. A genuine, unfettered right to send a substitute to perform the work in your place is one of the strongest indicators that you are self-employed rather than an employee. Yet many contractors either lack this right entirely or have a contractual clause that would not survive scrutiny. In this guide, I will explain exactly what the substitution test requires, how the tribunals assess it, and what you can do to strengthen your position.

Why Substitution Is So Important

The concept of personal service is fundamental to employment. An employee must perform the work personally — you cannot send someone else to your office job in your place. A self-employed person, by contrast, contracts to deliver a result. Whether they deliver it personally or through someone else is their choice.

This distinction is why substitution is considered a "knockout" factor in some IR35 analyses. If you have a genuine, unrestricted right to substitute and the client cannot unreasonably refuse, it is very difficult for HMRC to argue that the relationship is one of employment. As the tribunal in Express and Echo Publications v Tanton (1999) held, "an obligation to do work subject to the other party's control, with an unfettered right to provide a substitute, is inconsistent with a contract of employment."

However, the right must be genuine. A clause in a contract that exists only on paper — that would never be exercised and that the client would never accept — carries no weight. The tribunals have become increasingly sophisticated at distinguishing between genuine and sham substitution rights.

According to HMRC's Employment Status Manual on substitution, the key question is whether the worker is obliged to provide personal service, or whether there is a genuine right of substitution.

What Constitutes a Genuine Right of Substitution

For a substitution right to support an outside IR35 determination, it must meet several criteria:

The Right Must Be Contractual

The substitution right should be explicitly stated in your contract. While the tribunals look at reality over contracts, having the right in writing is the starting point. Without a contractual right, you are relying entirely on demonstrating that substitution is accepted in practice — a harder argument to make.

The Right Must Be Unfettered (or Near-Unfettered)

The client should not have an unrestricted right to reject any substitute you propose. It is reasonable for the contract to require the substitute to be "suitably qualified" or to possess necessary security clearances. It is not acceptable for the client to have the final say on whether any specific individual is acceptable — this gives them de facto control over personal service.

The distinction matters. A clause saying "the Contractor may substitute with a suitably qualified person" is strong. A clause saying "the Contractor may substitute with someone the Client approves" is weak, because the client could refuse every proposed substitute.

The Right Must Be Realistic

Could the right actually be exercised in practice? If the work requires such specialist knowledge that only you could do it, a substitution clause is less credible. If, however, there are other qualified professionals who could step in — perhaps someone from your own business, a subcontractor, or a colleague — the right is realistic.

The tribunals have found substitution rights genuine even where the contractor had not actually substituted, provided the right was realistic and could have been exercised. But actually having substituted significantly strengthens the position.

You Must Pay the Substitute

In a genuine business-to-business arrangement, if you send a substitute, you pay them from your company's income. The client pays your company the same fee regardless of who does the work. If the client pays the substitute directly, this suggests the client has more control over the individual worker, which points towards employment.

Common Substitution Mistakes

The Paper-Only Clause

The most common mistake is having a substitution clause in the contract that has never been tested and never could be. If you have worked for the same client for three years, five days a week, and have never mentioned substitution, the clause is paper-only. HMRC will argue — and the tribunals will likely agree — that the clause does not reflect reality.

To avoid this, consider proactively discussing substitution with your client, even if you do not exercise the right immediately. An email exchange where you mention the possibility and the client acknowledges it creates valuable evidence.

Client Approval Requirements

A clause requiring the client's approval effectively gives the client a veto over substitution. While "approval not to be unreasonably withheld" strengthens the clause, it still introduces an element of client control. The strongest clauses do not require approval at all — only notification and a requirement that the substitute be suitably qualified.

Ignoring the Practical Reality

Some contractors in highly specialised roles claim substitution rights that are implausible. If you are the only person in the UK with the specific combination of skills, security clearances, and client knowledge to do the job, claiming you could send a substitute is not credible.

In these situations, honesty is important. If substitution is not realistic for your engagement, do not rely on it as a factor in your IR35 assessment. Focus instead on other indicators like control, financial risk, and independence.

How Tribunals Assess Substitution

The tribunals take a multi-layered approach to substitution:

First, they look at the contract. Does it include a substitution right? What are the conditions? Is there a client veto?

Second, they look at practice. Has substitution ever been exercised? Has it been discussed? Would the client accept it?

Third, they assess genuineness. Is the right real, or is it a contractual fiction designed to create the appearance of self-employment?

The Dragonfly Consulting case is one of the clearest examples of substitution working in the contractor's favour. The consultant had actually sent a substitute on multiple occasions, with the client's knowledge and acceptance. The tribunal found this compelling evidence of genuine self-employment. For more on this and other key cases, see my article on IR35 case law decisions.

In contrast, in MDCM Ltd v HMRC, the contractor's substitution clause existed on paper but the tribunal found it was not a genuine reflection of the working arrangement. The contractor had never substituted and there was no realistic prospect of doing so.

Strengthening Your Substitution Position

If you are a contractor who wants to strengthen your substitution position, consider these practical steps:

Build a network of potential substitutes. Identify qualified professionals who could step in if needed. Keep their details on file and ensure they are aware of the possibility.

Discuss substitution with your client early. At the start of the engagement, mention your right to substitute and discuss the practicalities. Get the client's acknowledgement in writing (even an email is sufficient).

Exercise the right when opportunities arise. If you are unavailable for a period (holiday, illness, another commitment), offer a substitute rather than simply not working. Even a single instance of actual substitution is powerful evidence.

Keep records. Document any discussions about substitution, any occasions where you offered a substitute (whether or not the offer was taken up), and any actual substitutions that occurred.

Review your contract. Ensure the substitution clause is strong, does not give the client an unreasonable veto, and requires you (not the client) to pay the substitute. For guidance on contract drafting, see my guide on IR35-compliant contracts.

Substitution and the CEST Tool

HMRC's CEST tool includes questions about substitution, but its treatment of the subject has been criticised as oversimplistic. CEST asks whether you can send a substitute and whether the client can reject one, but it does not fully explore the nuances of genuine versus paper-only rights.

If CEST returns an "undetermined" result partly due to substitution ambiguity, it may be worth seeking a professional assessment that can analyse the specific circumstances of your engagement. For more on CEST's limitations, read my guide on IR35 and the CEST tool.

The Substitution Spectrum

Think of substitution as a spectrum rather than a binary:

Strongest position: You have exercised substitution, have a pool of available substitutes, pay them yourself, and the client accepts the practice without requiring approval.

Strong position: You have a clear contractual right, have identified potential substitutes, and the client acknowledges the right, even if it has not yet been exercised.

Moderate position: You have a contractual right subject to reasonable conditions (client notification, suitable qualifications), but have not discussed it with the client.

Weak position: You have a contractual right subject to client approval, have never discussed substitution, and the engagement is highly personal.

No position: Your contract requires personal service, or substitution is implausible given the nature of the work.

Understanding where you sit on this spectrum helps you assess the strength of the substitution factor in your overall IR35 determination. For a complete step-by-step assessment process, see my guide on how to determine your IR35 status.

As IPSE highlights, substitution remains one of the most contentious aspects of IR35 determination, with different tribunals weighing it differently depending on the facts.

Conclusion

Substitution is the most misunderstood IR35 test because it requires both contractual provision and practical reality. A clause on paper is not enough; genuine capability and willingness to substitute are what the tribunals look for. Invest time in building a credible substitution position — it could be the factor that keeps you outside IR35.

For help managing your contracting finances, Accounted tracks your income, expenses, and tax deductions throughout the year. Visit our features page to see how I can simplify your financial admin.

Penny, your AI bookkeeper, tracks your tax position in real time and flags opportunities to reduce your bill. Meet Penny →