Received an HMRC Investigation Letter — What to Do

Opening a letter from HMRC can make your stomach drop — especially when the words "tax investigation" or "enquiry" are staring back at you. It feels serious, and it can be hard to know whether you're in real trouble or whether this is routine. Take a breath. This guide explains exactly what this type of letter means, what you're required to do, and how to protect yourself.

What does this actually mean?

HMRC — His Majesty's Revenue and Customs, the UK's tax authority — has the legal power to investigate individuals and businesses to check that the right amount of tax has been paid. When they open a formal investigation, they are required by law to tell you in writing. This letter is that notification.

There are two main types of enquiry you might be facing:

• A routine enquiry (sometimes called a Section 9A enquiry): This is opened under the Taxes Management Act 1970 and can be triggered completely at random — it does not necessarily mean HMRC thinks you have done anything wrong. These are relatively common.
• A full or aspect enquiry: HMRC may be looking at a specific part of your tax return — for example, your self-employment income, property rental figures, or expenses — or they may be examining everything. A "full enquiry" means they are reviewing your entire return.

In more serious cases, you might receive a letter relating to a Code of Practice 9 (COP9) investigation, which is used when HMRC suspects serious tax fraud. If your letter mentions COP9 or fraud, you should seek specialist legal advice immediately — this is a significantly different process.

Do you need to act immediately?

Yes — but you do not need to panic. The key point is that HMRC letters almost always include a deadline, and missing it can make your situation worse.

• Response deadlines: HMRC typically gives you 30 days to respond to an enquiry letter or to provide documents they have requested. Check the date on the letter carefully and count from there.
• Requesting an extension: If 30 days is not enough time — for example, because you need to gather records or take legal advice — you can ask HMRC for more time. They will often agree to a reasonable extension if you contact them promptly.
• Do not ignore the letter: Ignoring correspondence from HMRC gives them the right to issue penalties and can lead to them making their own assessment of what they think you owe, which may be higher than the actual amount.

HMRC must open an enquiry within a set time window — usually 12 months after you submitted your tax return, though this period is longer if they suspect fraud or if information was not disclosed. If you think their enquiry may be out of time, a solicitor or tax adviser can check this for you.

Your next steps

1. Read the letter carefully. Note the type of enquiry, the tax year it covers, and any specific documents or information HMRC has asked for. Write down the deadline.
2. Gather your records. Locate the tax return the enquiry relates to, along with any bank statements, receipts, invoices, or other documents that support the figures you submitted.
3. Do not contact HMRC yet — not until you understand your position. Anything you say or send to HMRC can be used as part of their investigation. It is worth getting advice before you respond, even if the enquiry seems minor.
4. Seek professional advice. A solicitor or specialist tax adviser can review your return, assess how strong HMRC's position is, and help you draft a response that protects your interests.
5. Respond within the deadline — or request an extension in writing before it passes.

When should you speak to a solicitor?

You should speak to a solicitor as soon as possible if any of the following apply:

• The letter mentions fraud, serious non-compliance, or COP9
• HMRC is looking at multiple tax years or large sums of money
• You are a business owner or self-employed and the enquiry covers your trading income
• You have already missed a deadline or responded without taking advice
• You are unsure whether the figures in your tax return were correct
• You feel pressured or are struggling to understand what HMRC is asking for

Even in straightforward cases, having a solicitor handle correspondence with HMRC can prevent small mistakes from becoming bigger problems. Legal advice at an early stage is nearly always more cost-effective than trying to resolve things after they have escalated.

If you have received an HMRC investigation letter and are not sure where to start, CaseBridger is a free tool that gives you instant AI-powered guidance tailored to your situation and can help you find a qualified solicitor to support you through the process.