This will be the same tax office reference number as your PAYE reference number which HMRC use to track your employer’s payroll system. This means that your tax and National Insurance contributions are correctly related to your employment. Usually, you’ll normally find it on your payslip, P60, or P45. This consists of a three digit tax office code and a reference given to you by the employer. It is a number you will need if you have dealings with HMRC on a tax code, a refund or employment history in the UK.