During the registration process you'll need to enter your company registration number (CRN) and your Corporation Tax (CT) reference.
Your CRN is a unique reference allocated by the Registrar of Companies, to a company that is incorporated or registered under the UK Companies Acts.
The CRN is an 8 digit reference which is prefixed by the characters 'NI' (if registered in Northern Ireland) or 'SC' (if registered in Scotland).
Your CT reference or Company Unique Taxpayer Reference (UTR) will have been issued by HM Revenue and Customs (HMRC). It's made up of 10 numbers, for example 1234567890, and can be found on the Notice to deliver a company Tax Return (form CT603). It may also be found on other documents HMRC have issued.
Depending on the type of document issued the reference may be printed next to the headings 'Tax Reference', 'UTR' or 'Official Use'.