Will I have to pay back the tax if HMRC made the mistake, not me?

This is a nuanced area of UK tax law, and the answer depends on several factors, but the short version is that you may not have to repay tax if HMRC made the error and certain conditions are met. HMRC has a concession known as Extra Statutory Concession A19, which allows them to waive the recovery of underpaid tax in specific circumstances where the failure to collect the right amount was their fault rather than yours. For ESC A19 to apply, a few key conditions generally need to be satisfied. HMRC must have had all the relevant information they needed to collect the correct tax in the first place, meaning you provided everything accurately and on time. The error must have been on HMRC's part, such as failing to act on information they already held or making an administrative mistake. You must also have reasonably believed your tax affairs were in order, meaning you had no reason to suspect anything was wrong. Additionally, HMRC typically needs to have waited more than 12 months after the end of the tax year in which they received the relevant information before notifying you of the underpayment. If all these conditions are met, you can formally request that HMRC write off the debt under ESC A19, and they are generally expected to comply. However, it is important to understand that ESC A19 is not an automatic right, and HMRC does not always apply it without being challenged. You may need to write to HMRC explicitly requesting that they consider the concession and clearly explaining why you believe it applies to your situation. If HMRC refuses your request and you believe they are wrong, you can escalate the matter by complaining to HMRC directly, and if that does not resolve things, you can take your complaint to the Adjudicator's Office, which independently reviews complaints about HMRC's handling of tax matters. In more serious cases, the Parliamentary and Health Service Ombudsman can also be approached through your MP. There are situations where you will likely still have to pay even if HMRC made the mistake. If you knew or should reasonably have known that you were being undertaxed, HMRC is unlikely to waive the debt. For example, if your payslip clearly showed you were paying far less tax than expected, or if you received correspondence that should have alerted you to a problem, HMRC may argue you did not act in good faith. Similarly, if the underpayment relates to a very recent tax year and HMRC corrected it quickly, the 12-month rule under ESC A19 may not be satisfied. It is also worth noting that even when you do have to repay underpaid tax, HMRC is generally expected to allow you to pay it back in a manageable way rather than demanding a lump sum. They have a duty to treat taxpayers fairly, and if repayment would cause genuine financial hardship, you can negotiate a payment plan. In some cases involving very small amounts, HMRC may also decide it is not cost-effective to pursue the debt at all. If you find yourself in this situation, it is worth seeking advice from a tax professional or a free service such as TaxAid or Citizens Advice, who can help you understand your rights and the best course of action.