M Tax Code Marriage Allowance: Are You Claiming It?

Is your payslip showing a tax code with the letter M at the end, and you have absolutely no idea what it means or whether it's working in your favour? You are not alone, and the stakes are higher than most people realise.

The M tax code is one of the most quietly valuable codes HMRC can assign you, worth up to £252 a year in reduced tax. Yet millions of eligible married couples and civil partners have never claimed it, and a significant number who have claimed it are not confident the code on their payslip is actually correct. This article explains what the M tax code means, how Marriage Allowance works, and how to verify right now that you are receiving every penny you are owed.

What the Letter M Actually Means on Your Payslip

Tax codes are not random strings of numbers and letters. HMRC uses the letters to communicate specific adjustments to your Personal Allowance, and M is one of the most concrete of all of them.

When you see M at the end of your tax code, it means your spouse or civil partner has formally transferred 10% of their Personal Allowance to you. In the 2025-26 tax year, the standard Personal Allowance is £12,570. Ten per cent of that is £1,260. Your code therefore rises from the standard 1257L to 1383M, and HMRC instructs your employer to collect less tax from each payslip accordingly.

The result: you pay tax as though you earn £1,260 less per year. At the basic rate of 20%, that is a saving of £252 over the year, or £21 per month. It is not life-changing, but it is yours by right if you qualify, and HMRC will not chase you down to hand it over.

If you want to understand how tax codes are structured more broadly, What Does L Mean in a Tax Code? The Letter Explained covers the foundational mechanics behind the letter suffix system.

Who Qualifies for Marriage Allowance?

The rules are specific, and they catch people out in both directions: some assume they qualify when they do not, and others assume they do not when they absolutely do.

You and your spouse or civil partner qualify if all of the following apply:

• You are married or in a registered civil partnership (cohabiting couples do not qualify, regardless of how long they have been together)
• One of you earns below the Personal Allowance threshold, which is £12,570 for 2025-26, or has no income at all
• The other partner is a basic rate taxpayer, meaning their income falls between £12,571 and £50,270 (in England, Wales, and Northern Ireland)
• Neither of you pays Income Tax at the higher or additional rate

The lower-earning partner is the one who transfers the allowance. They lose 10% of their Personal Allowance, which only matters if they are not using it. The higher-earning partner receives it, expressed as the M suffix in their tax code.

One scenario that trips people up: if the lower-earning partner has any income, including a small part-time job, rental income, or a state pension, their unused Personal Allowance may be smaller than they think. It is worth checking your own position with a salary tax calculator before applying.

Scottish Taxpayers: A Small Wrinkle

If you live in Scotland and your income falls in the intermediate or higher rate bands under Scottish Income Tax, you may not benefit from Marriage Allowance in the same way. The allowance is still available, but the calculation differs slightly because Scottish rates and bands diverge from the rest of the UK. HMRC's own guidance confirms this, and it is worth double-checking if your payslip shows an S prefix on your tax code.

The Partner Who Gives: Tax Code N Explained

Every M tax code has a mirror image. The spouse or civil partner who transfers their allowance away will see an N appended to their tax code instead of the usual L.

So if a couple successfully claims Marriage Allowance, the payslips should show:

• Receiving partner: something like 1383M (standard allowance plus the transferred 10%)
• Transferring partner: something like 1131N (standard allowance minus the transferred 10%)

If you are in a couple where one of you believes you have claimed Marriage Allowance but one payslip shows M and the other still shows L, something has gone wrong. Either the claim did not go through, HMRC processed only one side, or one employer has not updated their payroll records. This is more common than it should be.

The fastest way to confirm both codes are correct is to check your tax code against your Personal Tax Account and ask your partner to do the same.

How to Claim Marriage Allowance (and Why HMRC Won't Do It Automatically)

HMRC does not apply Marriage Allowance automatically. It is an opt-in system, which means every eligible couple that has not claimed it is simply subsidising the Treasury without reward.

The application is made by the lower-earning partner, either online through HMRC's website or over the phone. Once approved, HMRC updates both partners' tax codes and notifies both employers. The change typically takes effect within a few weeks, though the precise timing depends on how quickly payroll processes the new code.

Claiming is free. No intermediary, no agent, no fee is required. Be wary of any website that charges a percentage of your refund to make this application on your behalf; you are paying for something HMRC provides at no cost.

If your circumstances change, for instance if the lower-earning partner's income rises above £12,570, you are responsible for cancelling the allowance. HMRC will not monitor this automatically, and if you fail to cancel it, you may end up with an underpaid tax bill. Underpaid Tax from a Wrong Tax Code: Who Really Pays? explains exactly how HMRC recovers that kind of shortfall.

Backdating: The Four-Year Window You Probably Do Not Know About

This is the section that tends to prompt a sharp intake of breath.

If you were eligible for Marriage Allowance in previous tax years but never claimed it, you can backdate your claim by up to four tax years. In practice, that means you can currently claim for:

• 2021-22
• 2022-23
• 2023-24
• 2024-25
• 2025-26 (current year)

The saving per year varies slightly because the Personal Allowance has changed over time, but across four full previous years plus the current year, a backdated claim could realistically recover more than £1,000 in overpaid tax.

That refund is paid directly to the lower-earning partner who makes the claim, not split automatically between the two. If you want to check whether you are owed a refund from previous years, How Long Does a Tax Refund Take From HMRC? covers the typical timelines once a claim is lodged.

Why Your M Tax Code Might Be Wrong

Having the M suffix in your tax code does not guarantee the number next to it is correct. Tax codes can contain errors for several reasons.

The allowance was claimed but the code was never updated. HMRC issues an updated tax code notice (a P2), but employers occasionally fail to implement it. Your payslip may show your old code for months after HMRC has already made the change on its system.

You have other income adjustments. If HMRC has also adjusted your code for untaxed savings interest, a benefit in kind from your employer, or an underpayment from a previous year, those adjustments interact with the Marriage Allowance uplift. The resulting number can be hard to verify without knowing what each component represents. How HMRC Calculates Your Tax Code: The Hidden Maths walks through how those layers stack up.

Your employer is using an old code. HMRC updates codes electronically through PAYE, but a small number of employers, particularly smaller firms running manual payroll, do not always receive or act on those updates in a timely manner. If you started a new job and your new employer was given an emergency code, the M suffix may have been lost entirely.

The claim was rejected without notification. If HMRC rejected the Marriage Allowance application, for instance because the lower-earning partner's income was too high, they do not always communicate this clearly. The applying partner may believe the claim succeeded when it did not.

The only reliable way to confirm your tax code is correct is to check it directly. You can check your tax code for free and compare it against what your payslip shows.

What to Do If You Have Multiple Income Sources

If you have more than one job, or income from both employment and, say, property rental, Marriage Allowance applies to your primary employment code only. HMRC will not split the transferred allowance across multiple income sources automatically.

If you have a second job already taxed at basic rate with a BR code, the Marriage Allowance on your primary code still applies. But if your total income from all sources starts to push you towards the higher rate band, you may lose entitlement to Marriage Allowance altogether. A multiple income tax calculator can help you model whether you remain in the qualifying band once all income is counted.

Checking Your Tax Code Today: A Two-Minute Task

You do not need to call HMRC, navigate a labyrinthine helpline, or wait for correspondence. Your current tax code is visible in three places: your most recent payslip, your Personal Tax Account on the HMRC website, and the most recent P2 tax code notice HMRC has issued to you.

Here is what to look for:

1. Find the tax code on your most recent payslip.
2. Check whether it ends in M.
3. If it does, confirm the number before the M is 1383 (for the standard 2025-26 tax year, assuming no other adjustments apply to you).
4. If the number differs significantly from 1383, there may be other adjustments applied, which may or may not be correct.
5. If there is no M and you believe you have claimed Marriage Allowance, check your Personal Tax Account or check your tax code to confirm what HMRC holds.

If you find an error, the process to correct it is straightforward. Change Tax Code Online With HMRC: A Step-By-Step Guide sets out exactly how to do it without spending an afternoon on hold.

The Couples Who Are Missing Out Right Now

HMRC has estimated that millions of couples are eligible for Marriage Allowance but have never claimed it. The most commonly cited reason is simply not knowing it exists. The second most common reason is assuming it is too complicated to apply for.

It is not. The application takes roughly five minutes online. The potential return, especially with four years of backdating, could be over £1,000 for a couple who has been eligible since 2021. That is not a marginal gain; for many households it covers a month of energy bills or a car service.

If you have a partner who earns less than £12,570, or has no income at all, and your own income sits comfortably in the basic rate band, the M tax code is yours by right. The question asked at the top of this article was whether you are actually claiming it. If you cannot see M on your payslip, the honest answer is: you are not.

The fix takes five minutes. Check your tax code now and find out exactly where you stand.

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