The Unique Taxpayer Reference (UTR) is only issued by HM Revenue & Customs (HMRC) when you, or your client, have registered for Self Assessment. It is made up of 10 numbers, for example 1234567890, and can be found on documentation issued by HMRC such as the Tax Return (SA100), a 'Notice to complete a Tax Return' (SA316) or a Statement of Account.
Depending on the type of document issued the reference may be printed next to the headings 'Tax Reference', 'UTR' or 'Official Use'.
Please note: If you're self-employed and you register online for business taxes, you will automatically be enrolled for each HMRC Online Service that you selected.
In all other cases, you'll need to register with HMRC for Self Assessment and enrol for the online service separately. If you have not yet registered with HMRC for Self Assessment this must be done to get the reference before you enrol for the online service.
For further information please see Registering for Self Assessment.