What W1, M1 and X usually mean
These markers usually mean week 1 or month 1 treatment. Payroll gives you only one slice of allowance and tax bands for the current pay period instead of using the normal cumulative position across the tax year.
Why it happens
- No P45 reached payroll in time.
- The starter checklist is still incomplete or not processed.
- You changed jobs, had a break, or payroll could not match the record cleanly.
Best next pages
Use Emergency Tax and Refund Checker to estimate whether too much tax may already have been taken, then use Why is my tax code BR? or the GOV.UK tax-code page if the code is something different.
Official sources: emergency tax codes and PAYE starter checklist guidance.