A UTR is the 10-digit number HMRC uses to identify you (or your company) in the Self Assessment and Corporation Tax systems. You get one automatically when you register as self-employed or for Self Assessment, and it never changes. You need it to file returns, use accounting software, register for the CIS, and talk to HMRC about your return.
Find it on previous returns, HMRC letters, or in your personal tax account. If you have genuinely lost it, HMRC can reissue it by post. Guard it like a password: a UTR plus basic personal details is enough for fraudsters to file fake refund claims in your name.
The UTR is ten digits, sometimes shown with a K on the end (1234567890K). You will be asked for it when filing returns, appointing an accountant, joining the CIS and setting up software. HMRC posts it within about two weeks of registering for Self Assessment, and it also appears in your personal tax account and the HMRC app under your Self Assessment section.
