Why Right of Substitution Determines IR35 Status

The right of substitution is one of the most commercially consequential clauses in a UK contractor engagement. In simple terms, it is the contractual right for a contractor to send someone else to do the work instead of attending themselves - and it is one of the three primary factors HMRC uses to determine whether an engagement falls inside or outside IR35. A contractor whose engagement includes a genuine, unqualified right of substitution looks less like an employee and more like an independent business offering services. An employee cannot send a substitute; a business can.

The word "genuine" matters enormously. HMRC and employment tribunals have consistently distinguished between a right of substitution that exists in the contract and one that exists in practice. A clause that says "the contractor may send a substitute" followed by a working relationship where the client clearly expects the named individual to show up every day and would refuse any substitute is not a genuine right of substitution. The contractual clause needs to reflect reality, not override it.

For staffing agencies placing contractors through personal service companies, the right of substitution clause affects both the IR35 determination and the agency's liability exposure if the determination is later challenged.

How Right of Substitution Works

A right of substitution clause in a contractor agreement typically states that the contractor may engage another suitably qualified person to fulfil the services under the contract, subject to client approval. The client approval proviso is acceptable as long as approval cannot be unreasonably withheld - the client can refuse a substitute who lacks the required qualifications, but they cannot refuse a substitute simply because they want the named individual.

For the right to be genuinely effective in an IR35 determination, three conditions should be present: the right must be stated clearly in the contract, the contractor must be responsible for paying the substitute from their own fees (not receiving a direct payment from the client), and the client must not have the power to reject the substitute arbitrarily.

HMRC's CEST (Check Employment Status for Tax) tool asks specifically whether the worker has an unrestricted right to send a substitute. A "yes" answer on an effective substitution right typically points toward outside IR35. A "no" answer, or a right that is heavily qualified by client veto, points toward inside IR35.

The practical reality is that most contractors in specialist technical or professional roles never exercise their right of substitution. That is fine - the right does not need to be exercised to be genuine. A contractor who has never sent a substitute but who could legally do so, and whose client would have to accept a suitably qualified substitute if one were sent, has a real right. A contractor who has never sent a substitute because they know the client would simply refuse any substitute has a nominal right that HMRC will look through.

A legal counsel at a technology staffing agency reviewed all contractor agreements following the 2021 IR35 reforms and found that 40% included substitution clauses that were qualified with "at the client's sole discretion" - a formulation that effectively nullified the right. She worked with the agency's external solicitors to revise the standard template to include an unqualified substitution right (subject only to qualifications the client could not reasonably withhold) and produced a plain-English explanation of the clause for both clients and contractors.

Right of Substitution in Practice

A senior IT contractor working through his PSC had included a right of substitution clause in his agency contract but had never discussed it with the client. When the client issued an inside-IR35 SDS, he challenged the determination and requested a review. As part of the challenge, he documented three specific instances where he had discussed sending a colleague to cover a day when he was unavailable, and the client had agreed in principle. He also noted that his contract with the agency specified his obligation was to deliver the services, not to attend personally. The client reviewed the SDS in light of the evidence and revised the determination to outside IR35, citing the genuine and demonstrable substitution right as a primary factor.