Ignoring an HMRC compliance check may result in penalties for failing to provide information or cooperate, and HMRC could make a tax decision using the information it already has. The letter should explain what HMRC needs, the deadline and how to respond, so contact them promptly if you need more time or cannot provide something. Tell HMRC about any genuine reasonable excuse and keep copies of your correspondence. If the check is complex or you disagree with HMRC’s position, consider speaking to a tax adviser and check the relevant guidance on GOV.UK.