M1 – This refers to Month 1. Either will appear at the end of your code (like 1257W1) to determine what you paid specifically during the period of uncertain tax information. HMRC sometimes uses X on UK tax codes which encompasses W1 and M1. Similarly, if your tax code has the letter...
If you have an emergency tax code, the code is usually followed by W1, M1, or X. You’ll need to contact HM Revenue & Customers (HMRC) in order to correct this, and you should then be paid back any tax you have overpaid. » MORE: How does the pay-as-you-earn system work?