Unique TaxPayer Reference (UTR) Number.
Your Unique Taxpayer Reference ("UTR" or "Reference" or "Self-Assessment reference") provided by Her Majesty's Revenue and Customers ("HMRC"). The number is made up of ten digits and is sometimes followed by the letter 'K', for example 1234567890K on HMRC documents and forms.
You can find your UTR on correspondence from HMRC, such as a previous Tax Return, Notice to complete a Tax Return or a Statement of Account. If you don't have a UTR you can contact your Tax Office and ask for a UTR - it can only be issued in the post to your home address. We cannot submit your return without both of these items.