The IR35 substitution clause: what makes it count
In short: employment needs personal service. If you have a genuine, unfettered right to send someone else to do the work, that is strongly inconsistent with employment. A right the client can veto at will, or that would never realistically be used, carries little weight.
Why substitution matters so much
The classic test from Ready Mixed Concrete (1968) starts with whether the worker must perform the work personally. In Express & Echo Publications v Tanton (1999) the Court of Appeal held that a contract allowing the worker to send someone else in his place was inconsistent with employment. That is why almost every outside-IR35 contract has a substitution clause — and why HMRC and tribunals look hard at whether it is real.
What weakens a substitution clause
- Client consent "at its absolute discretion". If the client can refuse any substitute for any reason, the right is largely illusory.
- Substitution only when you're ill or unavailable. A limited right to find cover is not the same as choosing who does the work.
- No mechanism. No mention of who pays the substitute, how they're vetted or how handover works.
- Reality contradicts it. The client interviewed you personally, the role needs security clearance only you hold, or everyone knows a substitute would be refused. Under Autoclenz v Belcher (2011), terms that don't reflect the true agreement can be disregarded.
Example wording
The Consultant may, with the prior written consent of the Client (which may be given or withheld at the Client's absolute discretion), provide a substitute.
The client can refuse anyone, for any reason. This gives little support to an outside-IR35 position.
The Consultant may at its discretion provide a substitute to perform all or part of the Services. The Client may only refuse a substitute on the reasonable grounds that they do not have equivalent skills, qualifications and any required security clearance. The Consultant remains responsible for the Services, will pay the substitute directly, and will bear any handover costs.
Clear, objective grounds for refusal, with the cost and responsibility sitting with your company.
Adapt any wording to your own situation — and make sure the client genuinely accepts it.
How to make substitution credible in practice
- Agree the clause with the end client, not just the agency — and check the upstream contract doesn't contradict it.
- Keep a record of suitable people you could send.
- If you have ever sent a substitute or used help at your own cost, keep the evidence.
- Make sure the role isn't defined around you personally (for example "the Consultant shall personally attend").
Substitution isn't everything
A strong clause helps, but tribunals weigh the whole picture — control, financial risk, integration and more. A clause that reality contradicts can even hurt, because it suggests the paperwork was written for IR35 rather than for the way the work is done.
Cases: Ready Mixed Concrete (South East) Ltd v Minister of Pensions [1968] 2 QB 497; Express & Echo Publications Ltd v Tanton [1999] ICR 693; Autoclenz Ltd v Belcher [2011] UKSC 41. General information, not legal or tax advice.
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