The IR35 Status Tests: Control, Substitution and Mutuality in Depth

Why the status tests still decide every IR35 case

There is no statutory definition of employment for tax purposes. IR35 asks a hypothetical question: if you stripped away the personal service company and the agency, would the contractor have been an employee of the end client? Answering that question means applying decades of case law, and almost all of it flows from one judgment. Tribunals, HMRC officers and status reviewers all reason from the same framework, so understanding it is the single most useful thing a contractor or hiring organisation can do before any contract is signed.

The framework matters in both directions. Under the off-payroll rules that apply to medium and large clients, the hiring organisation must apply these tests to reach its Status Determination Statement. For small clients, and for all clients once the small company threshold change from April 2026 moves more engagements back to the contractor side, the personal service company applies the same tests itself. Either way, the law being applied is identical.

The Ready Mixed Concrete three-stage framework

Ready Mixed Concrete v Minister of Pensions (1968) concerned a concrete delivery driver who owned his lorry, but the test it laid down has governed employment status ever since. MacKenna J held that a contract of service exists where three conditions are met. First, the worker agrees to provide their own work and skill in return for payment. Second, the worker agrees, expressly or impliedly, to be subject to the other party’s control to a sufficient degree. Third, the other provisions of the contract are consistent with it being a contract of service.

In modern shorthand, the first two conditions cover personal service, mutuality of obligation and control. Together they are called the irreducible minimum: if any one of them is missing, there cannot be employment, and the analysis stops there in the contractor’s favour. If all three are present, employment is possible but not yet established, and the tribunal moves to the third stage, weighing everything else about the relationship. Most contested IR35 cases are now won or lost at that third stage, because the irreducible minimum is present in the majority of professional contracting arrangements.

Personal service and the right of substitution

Employment requires personal service: the client hires you, not your company’s output. The classic way to negate personal service is a genuine right of substitution, meaning the company can send a suitably qualified replacement to do the work and the client must accept that replacement. Where a genuine, unfettered right exists, there is no obligation of personal service and the engagement falls outside IR35 on that ground alone.

The qualification matters. Tribunals distinguish a genuine right from a fettered one. A clause allowing substitution only with the client’s prior consent, which the client may withhold for any reason, is so heavily fettered that it usually amounts to personal service in practice. A clause that the client may refuse a substitute only on objective grounds, such as lacking the required qualifications or security clearance, is more defensible. Best of all is a right that has actually been exercised: a substitute sent, accepted and paid by the contractor’s company, with invoices to prove it.

HMRC and the tribunals also look hard at whether a substitution clause is a sham. If the work requires a named individual, if the client interviewed the contractor personally and would plainly never accept anyone else, or if the clause was inserted into the contract purely as IR35 dressing, the tribunal will give it little or no weight. A paper right that could never realistically be used does not negate personal service.

• Genuine right: the contractor’s company chooses, pays and remains responsible for the substitute, and the client can refuse only on objective grounds.
• Fettered right: substitution requires client consent that can be withheld at will, or applies only when the contractor is unable to work rather than unwilling.
• Sham right: the clause exists on paper but everyone involved knows the named individual must do the work personally.

Mutuality of obligation after PGMOL

Mutuality of obligation, usually shortened to MOO, describes the exchange of obligations between the parties: an obligation to provide work or pay, met by an obligation to perform it. For years, contractors argued that MOO meant something more demanding, such as an obligation on the client to keep offering work beyond the current assignment, and that its absence took them outside IR35.

The Supreme Court’s 2024 decision in PGMOL v HMRC largely closed that argument down. The case concerned part-time football referees engaged match by match, who could pull out of an appointment before the game without penalty. The court held that mutuality of obligation can exist within each individual engagement: once a referee accepted a match and officiated it for payment, the necessary exchange of work for wages was present during that engagement, even though neither side owed the other anything between matches. The court also confirmed that control does not require the ability to intervene during performance; a framework of sanctions and contractual obligations operating before and after the event can be enough.

The practical consequence for contractors is blunt. In a typical contract where the client pays for work done and the contractor performs it, MOO will almost always be present at the basic level the law requires. Arguing that a lack of MOO takes an engagement outside IR35 is now a weak strategy in most cases. The presence of work-for-payment mutuality does not make someone an employee by itself, but its absence can no longer be relied on as an easy escape route. The real battlegrounds have shifted to substitution, control and the third stage.

Control: how, what, when and where

Control is conventionally broken into four strands: what work is done, how it is done, when it is done and where it is done. Not all four carry equal weight. Control over how the work is performed is the most telling, because it goes to the heart of the master-and-servant relationship the older cases describe. A genuinely independent specialist is engaged for expertise the client does not have, so the client is in no position to direct the method. An employee, by contrast, can be moved between tasks and told how to carry them out.

Control over what is done deserves careful reading. Every client defines the deliverable, and that alone proves nothing. The question is whether the client can unilaterally move the contractor onto different work outside the agreed scope. If a project ends early and the contractor can be redeployed to whatever the client needs next, that looks like employment. If the engagement is tied to a defined statement of work and anything new requires a renegotiated contract, that points the other way.

When and where are the weakest strands, because modern working patterns blur them for employees and contractors alike. Fixed site hours on a construction project or mandated office days for security reasons say little about status when they would bind any worker, employed or not. Tribunals give these factors weight only where the client exercises genuine discretion over them, for example requiring attendance at internal meetings unrelated to the contracted deliverable. PGMOL added an important gloss here too: a contractual right of control matters even if it is rarely exercised in practice, so the terms of the contract on direction and supervision should be drafted and read with care.

The third stage: in business on your own account

Where personal service, MOO and sufficient control are all present, the case moves to Ready Mixed Concrete’s third stage: are the other provisions of the contract, and the wider circumstances, consistent with employment? The Court of Appeal’s 2022 decision in Atholl House, which concerned a radio presenter, confirmed that this stage is a genuine multi-factorial assessment, not a tick-box exercise. Crucially, the court held that a person’s wider professional life is relevant: someone carrying on an established business on their own account, with multiple clients and an independent professional reputation, may fall outside employment even where control and personal service exist on the particular engagement. The related Kickabout Productions appeal, heard alongside it, shows the same framework applied with a different result on its facts, which underlines how fact-sensitive the third stage is.

Factors the tribunals weigh at this stage include the following.

• Financial risk: fixed-price work, rectifying defects at your own cost, and the genuine possibility of making a loss on an engagement.
• Provision of equipment: supplying your own significant tools, software licences or premises rather than relying entirely on the client’s.
• Multiple concurrent or sequential clients, marketing for new work, and a business identity independent of any one engagement.
• Business overheads carried by the company: professional indemnity insurance, training paid from company funds, subcontractors engaged at the company’s expense.
• Absence of employee-style benefits: no holiday pay, sick pay, pension contributions, appraisals or notice-period protections from the client.
• Opportunity to profit from sound management, such as finishing fixed-price work efficiently or negotiating milestone payments.

No single factor is decisive. A tribunal paints a picture from all of them and asks whether, standing back, this looks like a business supplying services or an employee in disguise. Contractors who can evidence a real business, not just a payment vehicle, start the third stage with a substantial advantage.

Part and parcel: integration into the client’s organisation

A recurring third-stage theme is whether the worker has become part and parcel of the client’s organisation. Indicators of integration include appearing on internal organisation charts, managing the client’s employees, holding a client job title, attending staff social events and town halls, using a client email signature indistinguishable from staff, and accessing employee benefits such as subsidised facilities. None of these is fatal alone, and some are unavoidable, such as a client email address required for security. The pattern is what counts. A contractor engaged for a defined deliverable who sits visibly outside the staffing structure presents very differently at tribunal from one who has filled a line-management role for three years.

Length of engagement feeds into the same picture. There is no statutory time limit on outside-IR35 work, and a long engagement on a genuinely defined project can be defended. But successive renewals doing evolving business-as-usual work, where the contractor has effectively become a fixture, make the integration argument much harder to resist.

Written contract versus actual working practices

IR35 is decided on a hypothetical contract constructed from the whole arrangement, and tribunals look beyond the paperwork to how the engagement actually operates. Where the written terms and the day-to-day reality diverge, the reality generally wins. A contract stuffed with substitution clauses and statements of independence will not save an engagement where the contractor is, in practice, directed like an employee. Equally, a poorly drafted contract can undermine a genuinely independent working pattern, because the contractual right of control matters in itself, as PGMOL confirmed.

The defensible position is alignment: a contract that accurately records independent working practices, and working practices that honour the contract. That is why a professional review looks at both documents and behaviour together. An IR35 contract review that ignores the working reality is half a review. For hiring organisations issuing determinations, the same logic applies to the evidence behind each SDS, and the official guidance on how the off-payroll rules operate expects determinations to reflect actual arrangements, not template contracts.

Applying the tests to your own engagement

A sensible self-assessment works through the framework in order. Could your company genuinely send a substitute, and would the client accept one on objective grounds only? If yes, and you can evidence it, the analysis may end there. If not, examine control: who decides how the work is done, and could the client move you to other tasks? Then stand back and audit the third stage: financial risk, equipment, other clients, insurance, and how embedded you are in the client’s organisation. Write the answers down with evidence against each point, because an undocumented analysis is worth little when HMRC asks questions years later.

Status is rarely clear-cut, and the cost of getting it wrong falls asymmetrically: an incorrect outside determination can mean years of PAYE, NIC, interest and penalties. Where the picture is mixed, a specialist accountant can apply the case law to your specific facts, identify which factors need strengthening, and help you build the evidence file before anyone challenges it. The broader rules, rates and 2026 changes are covered in the complete IR35 guide, and the tools clients use to operationalise these tests are covered in the guide to CEST and Status Determination Statements.