Why These Tests Matter
IR35 has no single bright line. HMRC doesn't look at your job title, your company name, or how long you've been contracting. It looks at the nature of the working relationship — specifically, whether it looks more like self-employment or more like employment.
Three tests, developed through decades of case law, do most of the heavy lifting. Together they determine whether a contractor is genuinely in business for themselves, or is effectively a disguised employee.
- Right of Substitution — can you send someone else to do the work?
- Supervision, Direction & Control (SDC) — does the client control how you work?
- Mutuality of Obligation (MOO) — are you obliged to accept work, and is the client obliged to offer it?
No single test is automatically decisive, but courts and HMRC have consistently found that a genuine, unfettered right of substitution is the single strongest indicator of outside IR35 status. If you can send a substitute, you are not personally providing your services — and personal service is a cornerstone of employment.
HMRC assesses both your written contract and your actual working practices. A contract that says all the right things but doesn't reflect how you actually work will not protect you. Both must be consistent.
1. Right of Substitution
The right of substitution is the most powerful IR35 indicator. It asks a simple question: does the client need you personally, or do they just need the service delivered?
Employees cannot send someone else to do their job. Genuine contractors can. If your contract gives you a real, enforceable right to put a suitable replacement in your place — and the client can only object on reasonable grounds like suitability or security clearance — this is a strong indicator of outside IR35 status.
What makes a substitution clause strong?
- The right sits with your limited company, not you personally
- The substitute does not need the client's approval — only needs to meet reasonable skills/suitability criteria
- The cost of the substitute is met by you (your company), not the client
- The right is genuine and unconditional — not "subject to client agreement" in practice
What weakens a substitution clause
- "Subject to client approval" with no limit on what grounds approval can be refused
- Named individual requirements ("Jim Crawford must personally perform the services")
- A clause that exists on paper but has never been exercised or offered in the real relationship
- Clients who've confirmed in writing or in practice they would never accept a substitute
The right of substitution doesn't need to have been exercised for it to count. The fact that a genuine contractual right exists is sufficient — as confirmed in multiple tribunal cases. What matters is that the right is real, not theoretical.
Strong substitution — what it looks like
"My company has the right to provide a suitably qualified substitute at any time. The client may reasonably object only if the substitute lacks the necessary skills or security clearance. The cost of the substitute is borne by my company."
Weak substitution — what to watch for
"The contractor may not substitute without prior written consent from the client." — The word "consent" with no defined criteria gives the client a veto, making the right effectively meaningless.
Also watch for role descriptions that specify you personally ("the contractor agrees to personally carry out…") — this negates any substitution clause.
2. Supervision, Direction & Control (SDC)
SDC examines who controls how the work is done, not just what needs to be delivered. It is three distinct concepts, though they often overlap:
| Component | What it means | Employment indicator |
|---|---|---|
| Supervision | Someone watches over your work to ensure it is done correctly and to their standard | Being performance-managed, reviewed, or monitored on process |
| Direction | Someone tells you how to complete the work — the method, approach, or process | Being given detailed instructions on how to do the job, not just what to deliver |
| Control | Someone tells you what to work on and when | Being assigned tasks, expected to work specific hours, being on a rota or shift |
A genuine contractor is engaged to deliver an outcome. The client says what they want — a working software module, a completed tax return, a finished building phase — but they don't dictate how you go about it. Your expertise is precisely why you were hired.
By contrast, an employee is supervised, told how to work, and expected to follow the employer's processes. If a contractor's day-to-day reality looks like this, SDC points toward inside IR35.
SDC: inside vs outside in practice
No line management or appraisals
Flexible hours and location
Outcome-based — no process prescription
Not included in team meetings, rotas, or HR processes
Required to follow internal policies like an employee
Fixed hours, set desk, set location
Performance reviews or KPIs apply
Integrated into the client's team as if permanent
One of the most common IR35 red flags is integration — being treated as part of the permanent workforce. Using the client's email signature, appearing on their org chart, attending all-hands company meetings, being referred to as "our developer" — these are all factors HMRC considers when assessing control.
3. Mutuality of Obligation (MOO)
MOO has historically been the most misunderstood of the three tests — and its significance changed significantly following the 2024 Supreme Court PGMOL ruling.
What is MOO?
Mutuality of obligation refers to whether there is an ongoing, mutual commitment between the two parties: the client is obliged to offer work, and the contractor is obliged to accept it. This mirrors an employment relationship, where an employer offers regular work and an employee is expected to show up.
For genuine self-employment, neither party should be under such an obligation. A contractor takes on a specific project, delivers it, and the relationship ends. There is no expectation of further work, and the contractor is free to refuse future engagements.
What changed with PGMOL (2024)?
The Supreme Court's ruling in HMRC v Professional Game Match Officials Ltd (PGMOL) — a case about football referees — fundamentally changed how MOO is treated in IR35 cases.
The Court ruled that some degree of mutual obligation always exists within any engagement. The simple fact that a contractor accepts work and the client pays for it creates a "wage-work bargain" — a basic form of MOO. Arguing that MOO doesn't exist at all in an engagement is therefore no longer a viable strategy.
From 6 April 2026, MOO arguments carry significantly less standalone weight. The dominant tests are now Control and Right of Substitution.
How MOO still matters
MOO isn't irrelevant — it just can't be used as a standalone defence. Where MOO still has value is in demonstrating the absence of ongoing obligation between engagements. A contractor who works project-by-project with gaps between contracts, who genuinely has no guarantee of further work, and who is free to refuse further engagements, is in a stronger position than one on a rolling contract with continuous renewal.
What to document as evidence:
- Contracts have a defined end date (not open-ended)
- Gaps between contracts — even short ones — where no work or pay is provided
- No obligation to accept further work when offered
- No expectation from either party of ongoing work beyond the current contract
How HMRC Weighs the Tests Together
IR35 is not a checklist where you score points and pass or fail. It is a holistic assessment of the overall employment relationship. No single test automatically determines the outcome, and there is no fixed weighting.
That said, tribunals have consistently treated the tests as having a rough hierarchy in practice:
| Test | Practical weight | Direction since PGMOL |
|---|---|---|
| Right of Substitution | Highest — a genuine right is the strongest outside IR35 indicator | Unchanged |
| Control (SDC) | Very high — integration and day-to-day control are major factors | Increased |
| MOO | Supporting evidence only — can't be used as standalone defence | Reduced |
Other factors HMRC also considers — sometimes called secondary tests — include: financial risk (does the contractor bear business risk?), equipment provision (does the contractor provide their own tools?), part and parcel of the organisation (is the contractor integrated into the permanent workforce?), and exclusivity (does the contractor work for multiple clients?).
Working for more than one client simultaneously is a strong indicator of genuine self-employment — employees don't have other employers. While not one of the three primary tests, it carries weight and is easy to demonstrate if you keep records of concurrent or sequential client work.
Contract vs Working Practices — Why Both Matter
HMRC has the power to look beyond the written contract. The leading case is Ready Mixed Concrete v MPNI (1968), but modern IR35 cases confirm the same principle: if the contract says one thing and reality says another, HMRC will look at reality.
This matters because many contractors have "clean" contracts written by specialist IR35 lawyers — strong substitution rights, minimal control language — but work in ways that contradict those terms. Common examples:
- The contract has a substitution clause but the client has verbally confirmed they'd never allow a substitute in practice
- The contract says hours are flexible but the contractor works 9-5 Monday to Friday like every other employee
- The contract has an outcome-based scope but the contractor is line-managed and given daily task lists
The solution is to ensure your working practices are consistent with your contract and that you can evidence this. Keep records of project deliverables, avoid being included in permanent staff processes, and ensure your contract is renewed (not rolled over indefinitely) with a clear scope each time.
Quick Self-Assessment: Where Do You Stand?
Run through these questions honestly for your current engagement. More "yes" answers mean you're in a stronger outside IR35 position.
| Question | Outside IR35 | Inside IR35 |
|---|---|---|
| Does your contract include a genuine right of substitution? | Yes ✓ | No ✗ |
| Could the client refuse a substitute for any reason? | No ✓ | Yes ✗ |
| Does the client control how you do your work? | No ✓ | Yes ✗ |
| Are you required to work set hours or from a specific location? | No ✓ | Yes ✗ |
| Do you appear on the client's org chart or use their email address? | No ✓ | Yes ✗ |
| Does the client have a continuous obligation to offer you work? | No ✓ | Yes ✗ |
| Do you work for more than one client? | Yes ✓ | No ✗ |
| Do you bear financial risk (e.g. fix mistakes at your own cost)? | Yes ✓ | No ✗ |
HMRC's own CEST tool is unreliable for several reasons — it ignores Mutuality of Obligation entirely, returns "unable to determine" in roughly 20% of cases, and HMRC itself has refused to be bound by favourable CEST results in court. Use CEST as a starting point only. For any meaningful contract, a professional IR35 contract review is worth the cost.