The legal test that decides whether an HMRC penalty stands or falls — and the difference between an explanation HMRC accepts and one it rejects out of hand.
A reasonable excuse is the safety valve in the penalty system. HMRC's late filing and late payment penalties are largely automatic, but the law recognises that life sometimes makes a deadline impossible to meet. Where a genuine, unexpected event got in the way, a successful reasonable excuse cancels the penalty entirely. The catch is that HMRC applies the test strictly, and most rejected appeals fail because the "excuse" was really just an oversight.
There is no fixed list in law — HMRC and the tax tribunals judge each case on its facts. The working test is whether the event would have stopped a responsible person, who otherwise took reasonable care to meet their obligations, from doing so. Two things therefore matter: the event itself, and your conduct around it. Even a strong excuse fails if you delayed unreasonably once it had passed.
This is why timing is central. An illness in early January that lifts by mid-month may not excuse a return still unfiled in March. The excuse has to actually overlap with, and explain, the delay.
Commonly accepted excuses include a serious or life-threatening illness, an unexpected hospital stay, the death of a close relative or partner shortly before the deadline, a fire, flood or theft that destroyed your records, postal delays beyond your control, and HMRC's own service failures (for example the online filing system being down). Software or computer failures can count if genuinely beyond your control.
Commonly rejected excuses include forgetting the deadline, finding the online system too complicated, not getting a reminder, pressure of work, relying on an accountant or agent who let you down, and not having the funds to pay — although the reason you lacked funds (such as a sudden, unforeseeable event) might itself be an excuse.
Take Aisha, a sole trader whose 2024/25 return was due online by 31 January 2026. In mid-January she was admitted to hospital for an emergency operation and was unwell into early February. She files her return on 12 February 2026 and appeals the automatic late filing penalty.
| Factor | Aisha's position |
|---|---|
| Event | Emergency hospital admission (unexpected, outside her control) |
| Timing | Overlapped directly with the 31 January deadline |
| Action after recovery | Filed on 12 February, shortly after discharge |
| Likely outcome | £100 penalty cancelled |
HMRC is likely to accept this: the event was genuine and unforeseeable, it coincided with the deadline, and she acted promptly once able. Contrast that with filing in June with the same illness — the late filing would no longer be explained by the excuse. Check what the penalty would otherwise have been with the late filing penalty calculator.
You usually have 30 days from the date of the penalty notice to appeal. You can appeal online through your HMRC account, using form SA370, or by letter, setting out clearly what happened and the relevant dates. Be specific and factual — vague statements rarely persuade. If HMRC rejects the appeal, you can ask for a statutory review and, ultimately, take the case to the First-tier Tax Tribunal, which decides independently.
A reasonable excuse is not a sympathy plea — it is a factual case. What persuades HMRC is a genuine, unexpected event that lines up with the missed deadline, plus evidence that you acted as soon as you reasonably could.
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