The M1 tax code is an emergency non-cumulative tax code used by HMRC when they lack complete information about your employment or income. Unlike standard cumulative codes, M1 treats each pay period in isolation, which means you cannot carry forward unused personal allowance from previous months. This often results in overpaying tax until HMRC updates your record with the correct code.

Why HMRC assigns the M1 emergency tax code

HMRC applies an M1 suffix to your tax code when they need to calculate your tax on a month-by-month basis without reference to earlier periods in the tax year. The designation stands for Month 1, signalling that every pay period is treated as if it were the first month of employment.

Common triggers include:

  • Starting a new job without providing a P45 from your previous employer
  • Beginning employment for the first time and lacking a tax history
  • Returning to work after a period abroad or self-employment
  • Your employer submitting incomplete or missing starter information to HMRC
  • Receiving company benefits or a second income that HMRC has not yet reconciled

The M1 code is temporary by design. HMRC will replace it with a cumulative code once they receive sufficient details about your circumstances.

How the M1 tax code affects your take-home pay

Under a non-cumulative code such as M1, your employer calculates tax solely on what you earn in that specific pay period. Your personal allowance is divided equally across the year, and each month you receive one-twelfth of the annual allowance as a tax-free amount, regardless of what happened in earlier months.

Example using the standard personal allowance:

If your code is 1257L M1, you receive £1,047.50 tax-free each month (the current personal allowance of £12,570 divided by twelve). Any earnings above that threshold are taxed at the appropriate rate for that month alone.

By contrast, a cumulative code such as 1257L tracks your total earnings and total tax-free allowance from the start of the tax year. If you earned less in earlier months, the unused allowance rolls forward, reducing tax in later months when you earn more.

Why this matters:

  • If your income fluctuates, M1 can cause you to overpay tax in higher-earning months because you cannot offset lower earnings from previous periods
  • If you start a job part-way through the tax year, M1 ignores the personal allowance you have already accrued, potentially taxing you more heavily than necessary
  • Emergency codes do not account for other reliefs, allowances or adjustments HMRC may need to apply once they review your full record

The practical result is that many employees on M1 see a smaller net pay than they should, with the difference reclaimable once the correct code is in place.

Step-by-step guide to correcting your M1 tax code

Resolving an emergency tax code requires you to provide HMRC with the information they need to issue a cumulative code. Follow these steps in order:

Check your latest tax code notice

HMRC sends a PAYE coding notice whenever they change your code. Review this document to confirm the code currently in force and the reason HMRC applied M1. The notice will show whether the emergency code resulted from missing employment details, a recent job change, or another administrative gap.

Contact HMRC's Income Tax helpline

Call HMRC on 0300 200 3300 (Monday to Friday, 8am to 6pm). Have your National Insurance number, details of current and recent employments, and any P45 or P60 documents to hand. HMRC advisers can review your record in real time and issue a revised code if the information you provide is complete.

Submit your P45 to your new employer

If you started a new role without a P45, ask your previous employer to issue one if they have not already done so. Your new employer uses the P45 to report your previous earnings and tax paid in the current tax year, enabling HMRC to calculate your cumulative position accurately. If a P45 is unavailable, complete a starter checklist with your employer, selecting the statement that best describes your circumstances.

Update your Personal Tax Account online

Log in to your Personal Tax Account at gov.uk/personal-tax-account to check the employment and pension records HMRC holds. You can notify HMRC of a new job, confirm you have left a previous role, or report changes to benefits and expenses. HMRC will use this information to recalculate your code and send an updated notice to your employer.

Verify the new code appears on your payslip

Once HMRC issues a corrected code, they send it electronically to your employer or pension provider. Check your next payslip to confirm the M1 suffix has been removed and a cumulative code is now in use. If the emergency code persists despite HMRC confirmation, contact your payroll department to ensure the update has been processed in their system.

How long does the M1 tax code remain in force?

The M1 code is an interim measure and typically lasts no longer than one to two months, provided you or your employer supply the required information promptly. The exact duration depends on:

  • Speed of information submission: Providing a P45 or completing the starter checklist accurately can resolve the issue within weeks
  • HMRC processing workload: Peak periods such as the start of the tax year in April or the self-assessment deadline in January may extend processing times
  • Complexity of your tax affairs: Multiple employments, pension income, taxable benefits or other adjustments require HMRC to perform additional calculations before finalising your code

If you remain on M1 for more than two months, contact HMRC directly to identify any outstanding information or administrative delays.

Consequences of staying on M1 too long

Prolonged use of an emergency tax code almost always results in overpaying tax. Because M1 does not recognise unused personal allowance from earlier in the year, you forfeit the cumulative benefit that reduces tax as the year progresses.

What happens to overpaid tax:

  • HMRC reconciles your account once the correct code is applied, either during the tax year or after the year ends
  • Refunds are usually processed automatically and paid directly into your bank account or adjusted through your payroll
  • If HMRC does not issue an automatic refund, you can claim by contacting them or completing a P800 tax calculation if one is issued

The longer you remain on M1, the larger the potential overpayment and the longer you wait to recover those funds. Correcting the code promptly ensures your take-home pay reflects your true liability.

Preventing an M1 tax code in future

You can reduce the likelihood of being placed on an emergency code by maintaining accurate records and keeping HMRC informed of changes.

Keep HMRC updated with employment changes

Notify HMRC whenever you start or leave a job, begin drawing a pension, or take on additional employment. Do not rely solely on your employer to report changes. Use your Personal Tax Account or call the helpline to confirm HMRC has the correct details.

Retain and provide your P45 immediately

Always obtain a P45 when you leave a role and hand it to your new employer on or before your first payday. The P45 contains your earnings and tax paid to date, enabling your new employer to operate the correct code from the outset.

Review your tax code at the start of each tax year

Check your payslip and any coding notices you receive in April. Confirm that your code matches your circumstances and does not carry an M1 or W1 suffix. Early detection allows you to resolve discrepancies before they affect multiple pay periods.

Maintain copies of key documents

Keep P45s, P60s, coding notices and payslips in a secure location. These records are essential if you need to query your code or demonstrate your tax position to HMRC.

Claiming back emergency tax you have overpaid

If you have paid more tax than you owe while on an M1 code, you are entitled to a refund. HMRC processes most refunds automatically once they update your code and recalculate your liability for the year to date.

How refunds are issued:

  • If the correction happens during the tax year, your employer will usually adjust your next payslip to refund the overpayment through your salary
  • If the tax year has ended, HMRC will send a P800 tax calculation showing the amount due and arrange payment by cheque or bank transfer
  • You can also claim a refund by calling HMRC or submitting a claim through your Personal Tax Account if an automatic refund has not been issued

Refunds can take several weeks to process, particularly if HMRC needs to verify employment records or reconcile multiple income sources. Keeping accurate records and responding promptly to any HMRC queries will expedite the process.

Understanding other emergency tax code variations

The M1 suffix applies to employees paid monthly. HMRC uses similar non-cumulative codes for other pay frequencies:

  • W1 (Week 1): Applied to weekly-paid employees, operating on the same non-cumulative basis as M1
  • X suffix: Used when HMRC needs further information before issuing a standard code, often seen alongside emergency codes

All emergency codes share the same core characteristic: they treat each pay period independently and do not accumulate allowances or apply cumulative tax calculations. The steps to correct a W1 or X code are identical to those for M1.

Frequently asked questions

Can I continue working on an M1 tax code indefinitely?

You can remain employed on an M1 code, but you will likely overpay tax each month. HMRC expects employers and employees to provide the information needed to assign a cumulative code. If you take no action, HMRC will eventually reconcile your account at the end of the tax year, but you will have paid more tax than necessary in the meantime.

Will my employer automatically fix my M1 tax code?

Your employer can only operate the code HMRC provides. While your employer submits employment details to HMRC, they cannot change your tax code directly. You must contact HMRC or update your Personal Tax Account to trigger a code revision.

Does the M1 code affect my National Insurance contributions?

No. National Insurance is calculated separately from income tax and is not affected by whether your tax code is cumulative or non-cumulative. Your NICs are based solely on your earnings in each pay period, regardless of your tax code suffix.

What if I have two jobs and one is on M1?

HMRC allocates your personal allowance to one employment (usually your main job) and assigns a different code to your second income. If one job is on M1, contact HMRC to confirm which employment should receive your allowance and ensure both codes are correct. Multiple employments can complicate your tax position, so reviewing your codes regularly is essential.

How do I know if my M1 code is correct for my circumstances?

Compare the numeric part of your code (for example, 1257 in 1257L M1) with the current personal allowance published on gov.uk. If the figures do not align with your expected allowance, or if you believe HMRC should be applying adjustments for benefits, pensions or other income, contact HMRC to review your code.

Can I claim a refund for previous tax years if I was on M1?

Yes. You can claim overpaid tax for up to four previous tax years by contacting HMRC or submitting a claim through your Personal Tax Account. You will need to provide evidence of your earnings and tax paid, such as P60s and payslips, for the years in question.

When to seek professional advice

Most M1 tax code issues can be resolved by contacting HMRC directly. However, if your tax affairs involve multiple income sources, complex benefits, overseas earnings, or if you have been on an emergency code for an extended period without resolution, professional advice can save time and ensure accuracy.

We help clients across the West Midlands and throughout the UK navigate PAYE queries, tax code corrections and refund claims. If you are uncertain about your tax position or need support reconciling your records with HMRC, get in touch for a consultation.

Key takeaways

  • The M1 tax code is a temporary emergency code that treats each month independently, preventing you from carrying forward unused personal allowance
  • You are typically placed on M1 when HMRC lacks complete employment information, often when starting a new job without a P45
  • Correcting the code requires you to contact HMRC, provide missing documentation, and verify the update appears on your payslip
  • Overpaid tax is refunded automatically once HMRC applies the correct code, either during or after the tax year
  • Prevent future emergency codes by keeping HMRC informed, retaining key documents, and reviewing your tax code regularly

Understanding how the M1 tax code operates and taking prompt action to resolve it ensures you pay the correct amount of tax and receive your full entitlement each payday. For further guidance on PAYE, tax codes or any aspect of personal tax compliance, explore our knowledge hub or contact our team.