HMRC has issued a tax warning to millions of UK taxpayers who may have paid the wrong amount of income tax in the 2025–26 tax year, urging people to check their tax records before the self-assessment and reconciliation deadlines. HM Revenue & Customs sends out P800 tax calculation letters annually to individuals whose tax payments do not match what they actually owe. If you have received one — or suspect your tax code is wrong acting quickly can save you money or prevent an unexpected bill.
HMRC Tax Warning: Why People Pay the Wrong Tax
The HMRC tax warning applies to a wide range of common scenarios that cause underpayments or overpayments. The most frequent cause is an incorrect tax code, which employers and pension providers use to calculate how much tax to deduct from each pay cheque. A wrong tax code can mean HMRC collects too much or too little throughout the entire tax year without the taxpayer noticing.
Other common triggers include starting or leaving a job mid-year, receiving multiple income sources, claiming the wrong number of allowances, or failing to notify HMRC of a change in circumstances. Reports suggest that in any given tax year, more than 5 million UK workers receive an incorrect tax code at some point. HMRC encourages all employees and pensioners to check their tax code on every payslip.
HMRC Tax Warning: How to Check If You Paid the Wrong Tax
Acting on the HMRC tax warning starts with checking your personal tax account. Every UK taxpayer can access their tax summary, current tax code, and payment history by logging into the HMRC portal at gov.uk/personal-tax-account using their Government Gateway credentials. The account clearly shows whether you have overpaid or underpaid tax for the current and previous tax years.
You should also check:
- Your tax code on your payslip or pension statement (e.g., 1257L is the standard code for most employees)
- Your P60 (issued by your employer at year-end) for total tax paid
- Any P800 letter sent by HMRC, which confirms whether you are due a refund or owe additional tax
- Your employer’s RTI submissions, which HMRC matches against your self-reported income
If your tax code contains letters like K, BR, OT, or NT, it may indicate a non-standard arrangement that warrants a closer review.
HMRC Tax Warning: What Happens If You Overpaid Tax
The HMRC tax warning carries good news for overpayers — HMRC will typically issue a refund automatically once it identifies the discrepancy through its end-of-year reconciliation process. Most refunds arrive via a P800 letter, after which you can claim the money online within 45 days or wait for a cheque to be posted to your registered address. HMRC usually completes reconciliation for the previous tax year between June and November.
If you believe you have overpaid but have not received a P800, you can submit a formal reclaim directly through your personal tax account or by calling HMRC on 0300 200 3300. The standard time limit to claim a tax refund is four years from the end of the relevant tax year — meaning claims for the 2021–22 tax year must be submitted before April 5, 2026. Claims submitted after this window are not publicly guaranteed to be honoured.
HMRC Tax Warning: What Happens If You Underpaid Tax
The HMRC tax warning is equally important for underpayers, as HMRC will recover any shortfall — with or without a reminder. For underpayments below £3,000, HMRC typically recoups the amount by adjusting your tax code in the following tax year, spreading the recovery across 12 months of payroll. For larger underpayments, HMRC may issue a formal demand or expect payment via self-assessment.
| Underpayment Amount | HMRC Recovery Method |
|---|---|
| Under £3,000 | Adjusted tax code in the next tax year |
| £3,000 and above | Direct payment request or self-assessment bill |
| Persistent underpayment | Formal compliance enquiry or penalty notice |
Ignoring an underpayment notice is not advisable — HMRC charges interest on late tax payments at a rate currently set above 7% per annum. Penalties for deliberate non-disclosure can reach 100% of the tax owed in the most serious cases.
HMRC Tax Warning: Common Tax Code Mistakes to Watch For
The HMRC tax warning specifically highlights tax code errors as the single most preventable cause of wrong tax payments. Your tax code should reflect your Personal Allowance (£12,570 for most people in 2025–26), any adjustments for benefits in kind, and any additional income sources. If your code reads BR (Basic Rate on all income) or 0T (zero allowance), you may be paying too much tax immediately.
Common tax code errors include:
- Emergency tax codes applied when starting a new job without a P45
- Duplicate allowances claimed across two concurrent employments
- Benefits in kind (company car, private medical insurance) not correctly factored in
- Marriage Allowance transfers not applied despite a valid election
- State Pension not accounted for in a pensioner’s tax code
Correcting a tax code requires contacting HMRC directly — either online via the personal tax account, via the HMRC app, or by phone.
HMRC Tax Warning: How to Contact HMRC About a Tax Error
If the HMRC tax warning applies to your situation, you have several official routes to resolve the issue without delay. The fastest method is through the HMRC personal tax account at gov.uk/personal-tax-account, where you can update income details, correct tax code information, and send secure messages. The HMRC app also allows users to check their tax code and request corrections directly from a mobile device.
For those who prefer telephone contact, HMRC’s Income Tax helpline operates at 0300 200 3300, Monday to Friday, 8am to 6pm. Written correspondence should be directed to HMRC, Pay As You Earn and Self Assessment, BX9 1AS — though response times by post can exceed eight weeks during peak periods. Reports suggest that using the online portal or app results in significantly faster resolution than written correspondence.
HMRC Tax Warning: Self-Assessment Taxpayers
The HMRC tax warning carries additional urgency for self-assessment taxpayers, who carry personal responsibility for declaring all income accurately each year. The 2025–26 self-assessment return must be filed online by January 31, 2027, with any tax owed paid by the same date. Missing this deadline triggers an automatic £100 fine, with further daily penalties accruing after three months of non-filing.
Self-employed individuals, landlords, company directors, and those with income over £100,000 are all required to file a self-assessment return regardless of whether HMRC sends a prompt. If you are unsure whether you need to file, HMRC’s online eligibility checker at gov.uk/check-if-you-need-tax-return takes fewer than five minutes to complete.HMRC Tax Warning: Deadlines You Must Not Miss
| Action | Deadline |
|---|---|
| Claim overpaid tax for 2021–22 | April 5, 2026 |
| Register for self-assessment (2025–26) | October 5, 2026 |
| File paper self-assessment return | October 31, 2026 |
| File online self-assessment return | January 31, 2027 |
| Pay any tax owed for 2025–26 | January 31, 2027 |
HMRC does not issue personalised reminders to every taxpayer, so maintaining awareness of these dates is the individual taxpayer’s own responsibility. Setting calendar alerts at least 30 days before each deadline gives adequate time to gather documents and avoid last-minute errors.
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