See W1, M1 or 0T on your payslip and you are on an emergency code — paying more tax than you should until HMRC catches up.
The first sign of an emergency tax code is usually a payslip that is lighter than expected. The code itself is easy to miss, but the suffix on it (W1, M1 or 0T) is doing all the damage, taxing you as though the rest of the tax year never happened.
HMRC tells your employer which tax code to use. When that instruction is missing, your employer falls back to an emergency code. The defining feature is that most emergency codes are non-cumulative: instead of looking at everything you have earned and paid since 6 April, they treat each pay period in isolation.
There are two main emergency codes for 2025/26:
Suppose Nadia starts a new job in July 2025 earning £36,000 a year (£3,000 a month) but cannot provide a P45, so she is placed on 0T.
Under 0T, her whole £3,000 monthly pay is taxed with no allowance:
| Code | Monthly tax |
|---|---|
| Correct 1257L (cumulative) | ≈ £390 |
| Emergency 0T | ≈ £600 |
| Extra deducted per month | ≈ £210 |
On 0T she pays around £210 a month too much, because the code ignores the £12,570 of tax-free allowance she is entitled to. Once she hands over her starter checklist and HMRC issues the correct cumulative 1257L code, her employer's payroll recalculates the whole year to date and the overpaid tax is credited back through her later payslips, all within the same tax year. She can confirm which code is live by choosing to check her tax code rather than waiting for payroll to tell her.
Had Nadia been on the gentler 1257L W1/M1 code instead, she would still have received a month's worth of allowance each pay period, so the overpayment would have been smaller. The harm from W1/M1 comes not from losing the allowance but from the code being non-cumulative: it cannot reach back to recover allowance she did not use earlier in the year, so any overpayment sits unrefunded until the code becomes cumulative again.
The fix is almost always to supply the missing information:
Pension withdrawals are a special case worth flagging on their own. When you take a first flexible lump sum, the provider usually has no cumulative earnings data, so it applies emergency tax and effectively annualises that single payment, treating it as though you will receive the same amount every month. A modest one-off withdrawal can therefore be taxed as if it were part of a six-figure annual income, producing an overpayment that runs into thousands of pounds. Reclaim forms P55, P53Z and P50Z let you recover the overpayment without waiting for year-end, with the right form depending on whether you have fully emptied the pension and whether you have other taxable income that year.
An emergency tax code is HMRC being cautious, not punitive. Give it the information it is missing and the code corrects itself.
If you are a sole trader earning only self-employed income, emergency tax codes do not apply to you, because you pay Income Tax through Self Assessment (and, from April 2026, Making Tax Digital quarterly updates) rather than PAYE. However, if you also have an employed role or draw a pension, those income streams run through PAYE and can be hit by an emergency code in exactly the same way as for anyone else.
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