If you have spotted ‘1257L M1’ or simply ‘M1’ on your payslip and are not quite sure what it means, you are in the right place. The M1 tax code is an emergency tax code issued by HM Revenue and Customs (HMRC) on a temporary basis when they do not yet have enough information to assign you the correct, cumulative tax code.
It is not a penalty, and it does not mean you have done anything wrong. However, it does usually mean you are paying more Income Tax than you should. Understanding what it is, why you have it, and how to resolve it quickly can put more money back in your pocket each month.
In this comprehensive guide you will learn:
- What the M1 tax code means in plain English
- Why HMRC assigns it and the most common triggers
- How it affects your monthly take-home pay, with a worked example
- What 1257L M1 looks like on your payslip
- How to change your M1 tax code as quickly as possible
- What happens after your code is corrected, including tax refunds
- Other related emergency tax codes to be aware of
What Is the M1 Tax Code?
The M1 tax code is an emergency tax code used by HMRC under the Pay As You Earn (PAYE) system. The ‘M1’ suffix stands for ‘Month 1’ and signals to your employer that your Income Tax must be calculated on a non-cumulative, month-by-month basis rather than cumulatively across the whole tax year.
Under a standard cumulative tax code such as the most common 1257L for the 2025/26 tax year your employer considers your total earnings and total tax paid since 6 April each time they run payroll. This means any unused personal allowance from earlier months carries forward and reduces your tax bill in later months.
With the M1 tax code, that backward-looking calculation is switched off. Each month is treated in complete isolation, as though it were the very first pay period of the tax year. You receive only one-twelfth of your annual personal allowance (£12,570 ÷ 12 = £1,047.50 for 2025/26), regardless of how long you have been employed or how little you earned previously in the year.
What Does Tax Code M1 Mean?
In practical terms, tax code M1 means your employer is treating every pay day as a fresh start. Two important consequences follow from this:
- Your unused personal allowance from previous months is ignored. If you were not working for the first six months of the tax year, you would ordinarily have built up six months’ worth of untaxed allowance. The M1 code wipes this out.
- You are very likely to overpay Income Tax compared to someone on a cumulative code with the same salary.
HMRC applies the M1 code because they need some tax to be collected in the interim but they have not yet received the details needed to calculate your tax correctly. It is essentially a precautionary holding measure.
Why Have I Been Given the M1 Tax Code?
There is rarely a single cause. HMRC applies the M1 code whenever they lack sufficient information to issue an accurate cumulative code. The most common reasons include:
Starting a New Job Without a P45
If you begin a new role and cannot provide your employer with a P45 — the document issued by your previous employer showing your pay and tax details for the current year — HMRC will not have the data they need. Your employer will ask you to complete a Starter Checklist (formerly called a P46). Depending on which statement you tick on the checklist, you may be placed on an emergency code such as 1257L M1.
Changing Jobs Mid-Year
Even if you do have a P45, a delay in processing or an administrative oversight can mean HMRC temporarily uses an emergency code whilst they update their records. HMRC’s systems can take up to 35 days to update after receiving new information from your employer.
Having Multiple Jobs
If you work for more than one employer simultaneously, HMRC may apply an emergency code to a secondary job whilst they calculate how to split your personal allowance. Typically your main job will receive the standard 1257L code, and your secondary job will receive either a BR code (Basic Rate, 20%) or an M1 variant.
Taking a Pension Lump Sum
If you withdraw a taxable lump sum from a defined contribution pension — for example, under pension freedoms rules — your provider will often apply the M1 code in the absence of a specific tax code from HMRC. This can lead to significant overtaxation because HMRC’s system initially treats the lump sum as though it is a regular monthly payment (and therefore projects it as a much higher annual income).
Changes in Employment Status
Moving from self-employment into PAYE employment, returning to work after a period of unemployment or sick leave, or transitioning from the gig economy (for example, moving from platform-based freelance work to a salaried role) can all trigger the M1 code whilst HMRC catches up with your new circumstances.
Missing or Outdated Tax Information
If you have not updated your details with HMRC for example, following a change of address, marital status, or income source they may default to the M1 emergency basis until the records are refreshed.
How Does the M1 Tax Code Affect Your Income?
The core impact is straightforward: you are likely to pay more Income Tax than you actually owe in the months you are on the M1 code. Here is why this happens and how significant the difference can be.
Cumulative vs Non-Cumulative Tax Calculation
Under a cumulative PAYE calculation your employer looks at the whole tax year to date. If you were unemployed from April to October and then started a new job in November, you would have built up seven months of unused personal allowance (7 × £1,047.50 = £7,332.50). A cumulative code applies this unused allowance against your November wages, meaning you could earn quite a lot before paying any tax.
Under the M1 code, none of that previous context matters. Your employer simply grants you £1,047.50 of allowance for November and taxes the rest at the standard rate.
Worked Example (2025/26 Tax Year)
Suppose you were unemployed from 6 April 2025 and started a new job on 1 January 2026, earning £2,500 per month. Here is how your January tax bill differs under each code:
| Scenario | Tax-Free Allowance Applied | Taxable Income | Income Tax Deducted |
| Standard 1257L (cumulative) | £12,570 (full year’s unused allowance) | £0 | £0 |
| M1 Emergency Code | £1,047.50 (one month only) | £1,452.50 | £290.50 |
| Difference (overpayment) | — | — | £290.50 |
As the table above illustrates, you could overpay nearly £291 in a single month simply because of the M1 code. If the code persists for several months for example, if you remain on it for the full remaining five months of the tax year the total overpayment could reach over £1,450. This is money that HMRC does owe you back, but reclaiming it takes time.
What Does the M1 Tax Code Look Like on Your Payslip?
On your payslip, the tax code is usually displayed in a dedicated field near your employee number or name, often labelled ‘Tax Code’ or ‘T. Code’. For the 2025/26 tax year, the M1 code will appear in one of the following formats depending on your region and tax situation:
| Tax Code on Payslip | What It Means |
| 1257L M1 | Standard personal allowance (England/Northern Ireland), applied on a Month 1 emergency basis |
| S1257L M1 | Scottish taxpayer — same emergency basis, but Scottish Income Tax rates apply |
| C1257L M1 | Welsh taxpayer — same emergency basis, Welsh rates (aligned with England for 2025/26) |
| SK1257 M1 | K code: your deductions exceed your allowances; still applied on Month 1 basis |
| 0T M1 | Zero personal allowance — all income taxed; Month 1 basis applied (see 0T tax code) |
The crucial identifier to look for is the ‘M1’ suffix. This alone confirms your tax is being calculated on a non-cumulative, emergency basis. The numbers before it tell you the amount of personal allowance you are receiving (if any), and any letter prefix tells you your tax region.
How Can I Change My M1 Tax Code?
The good news is that the M1 tax code is temporary and usually straightforward to resolve. Your aim is to give HMRC the information they need so that they can issue a corrected, cumulative PAYE Coding Notice (also known as a P2) to both you and your employer.
Step 1: Provide Your P45 to Your New Employer
If you have recently changed jobs, the fastest route is to hand your new employer your P45 from your previous job as soon as possible. The P45 contains your tax reference, the tax code in use at your last job, and your cumulative pay and tax figures for the current tax year. Your employer forwards this to HMRC, who can then update your code promptly.
If you do not have a P45 for example, because this is your first job, you have been unemployed, or your previous employer has not yet issued one complete the Starter Checklist provided by your new employer instead. Answer the questions honestly, as your answers determine which interim code is applied.
Step 2: Update HMRC via Your Personal Tax Account
The most reliable self-service route is to log in to your Personal Tax Account on GOV.UK or through the HMRC mobile app. Once logged in you can:
- Check your current tax code and see the breakdown
- Update your employment details (new employer, new income, changes to benefits)
- Notify HMRC of changes to your income, pensions, or taxable benefits
- Request a code review if you believe your code is wrong
After you update your details, HMRC will review your records and issue a revised cumulative code. Your employer will receive a P2 Coding Notice and apply the new code in the next available payroll run.
Step 3: Contact HMRC by Phone
If you are unable to resolve the matter online, you can call the HMRC Income Tax helpline:
- Telephone: 0300 200 3300
- Hours: Monday to Friday, 8am to 6pm
- Have ready: your National Insurance number, the tax code on your payslip, and any P45 or P60 documents
HMRC agents can update your record directly during the call and arrange for a corrected code to be issued without delay.
What Happens After Your M1 Tax Code Is Corrected?
Once HMRC issues your new cumulative code and your employer applies it, two things will happen automatically:
- Going forwards, your Income Tax will be calculated correctly on a cumulative basis, taking into account everything you have earned and paid since 6 April.
- If the cumulative calculation shows you have overpaid tax during the period you were on the M1 code, the excess will be refunded directly through your payroll usually as a higher net pay in your very next payslip.
This is one of the advantages of the PAYE system: your employer’s payroll software handles the reconciliation automatically, so you do not have to submit a formal tax return in most cases.
What If You Stay on the M1 Code Until the End of the Tax Year?
If the M1 code remains in place until 5 April 2026 the end of the 2025/26 tax year HMRC will carry out a post-year-end reconciliation once your employer has submitted their final PAYE submissions. HMRC will then issue a P800 tax calculation letter (typically in June or July following the end of the tax year) showing whether you have overpaid or underpaid.
If the P800 shows a refund is due, you must claim it it will not be paid automatically. The simplest way is via your Personal Tax Account or the HMRC app, where you can request payment directly to your bank account. Alternatively, HMRC will issue a cheque if you do not claim online within 45 days.
M1 Tax Code and Pension Withdrawals
Pensioners taking flexible withdrawals from a defined contribution pension face a particular risk with the M1 code. When you withdraw a lump sum from a pension for the first time in a tax year and your provider does not hold a tax code from HMRC, they are legally required to deduct tax on a Month 1 emergency basis.
This means HMRC’s system temporarily treats the lump sum as though it is a recurring monthly income. For example, a single withdrawal of £10,000 would be taxed as though your annual income were £120,000 potentially attracting the 40% or even 45% higher rate — rather than being taxed correctly against your actual annual income.
The overpayment can be substantial. Fortunately, HMRC provides specific repayment forms for pension overtaxation:
- Form P50Z — if you have emptied your pension pot and have no other income
- Form P53Z — if you have taken a small pots payment or a trivial commutation lump sum
- Form P55 — if you have only partially withdrawn from your pension and have no other taxable income
Submitting the appropriate form can secure a refund within 30 days, rather than waiting until after the tax year end.
Other Emergency Tax Codes You May Encounter
The M1 code is one of several emergency or non-standard tax codes used by HMRC. It is useful to understand the full picture:
| Tax Code | Meaning | Who It Applies To |
| 1257L W1 | Week 1 emergency — same as M1 but for weekly payroll | Weekly-paid employees |
| 1257L M1 | Month 1 emergency — non-cumulative monthly calculation | Monthly-paid employees/pensioners |
| 1257L X | Non-standard pay periods — non-cumulative basis | Irregular pay periods |
| BR | Basic Rate — all income taxed at 20%, no personal allowance | Secondary jobs/pensions |
| 0T | Zero personal allowance — all income taxed at applicable rates | No allowance situations |
| 0T M1 | Zero allowance on a Month 1 emergency basis | No allowance + emergency basis |
| K codes | Deductions exceed allowances — effective negative allowance | Benefits-in-kind, underpaid tax |
Key Takeaways: M1 Tax Code at a Glance
| Feature | Detail |
| What it is | A temporary, non-cumulative emergency tax code |
| What ‘M1’ stands for | Month 1 — your tax is calculated as if each month is the first of the year |
| Personal allowance applied | £1,047.50 per month (£12,570 ÷ 12) for 2025/26 |
| Main impact | Likely Income Tax overpayment; unused allowance from earlier in the year is ignored |
| Most common trigger | Starting a new job without submitting a P45 or Starter Checklist |
| How to fix it | Provide P45 to employer; update details via Personal Tax Account or call 0300 200 3300 |
| Overpaid tax | Refunded automatically through payroll once corrected, or via P800 after year end |
| 2025/26 standard code | 1257L M1 (England/N. Ireland); S1257L M1 (Scotland); C1257L M1 (Wales) |