Emergency tax codes appear when your employer does not have enough information to tax you correctly, typically in a new job without a P45, or after starting to draw a pension. The code (such as 1257L W1 or M1) gives you the standard allowance but calculates each payday in isolation rather than cumulatively, so it cannot refund earlier overpayments automatically.
Emergency codes usually fix themselves once HMRC matches your records, and any overpaid tax comes back through your pay or as a refund. If a W1 or M1 suffix survives more than a couple of months, contact HMRC rather than waiting. Our tax code guides show exactly what each emergency code means.
Starting a new job in July with a £4,000 first payslip (including back pay), an M1 code taxes that month in isolation as if you earn £48,000 a year, so you overpay compared with the cumulative calculation that knows you had no pay in April, May or June. Once the proper code arrives, payroll refunds the difference automatically in your next payslip.
