A UTR number (Unique Taxpayer Reference number) is a ten-digit code that is used to identify you or your business. You can apply for your UTR number online with HMRC directly; It takes up to 10 working days to get a new UTR number. After you receive it, you need to activate it with an activation code, which will take a further week to arrive. Additionally If you can't remember it, take a look at any previous tax returns and other documents from HMRC.