In the United Kingdom and the Republic of Ireland, a P45 is the reference code of a form titled Details of employee leaving work. The term is used in British slang as a metonym for termination of employment. The equivalent slang term in the United States is pink slip.
In the United Kingdom
In the UK, the front section, Part 1, is given by the old employer to HM Revenue and Customs, who then record the pay and tax details on to the individual's taxpayer record. Part 1A is to be retained by the employee, Part 2 retained by the new employer, and Part 3 taken by the new employer and sent to their tax office. The P45 contains details of earnings and tax paid during the tax year (tax paid in previous years is detailed on the P60 for that year).
The "P" code refers to documents in the PAYE series, in the same way that self-assessment documents are prefixed "SA" (e.g. SA100 - Individual tax return) and Tax credits paperwork is prefixed "TC" (e.g. TC600 - Tax credits application).
In the Republic of Ireland
In the Republic of Ireland, P45 is also used as a reference to a form used by the Revenue Commissioners with the same function. The administrative procedures in this area are similar; however, the form itself is different in design from the UK version. The form is planned to be eliminated from 1 January 2019, with a move instead towards real-time reporting of tax details to Revenue. 
- "PAYE forms - P45, Starter Checklist, P60 and P11D". HM Revenue & Customs. Retrieved 2013-05-18.
- "When an employee leaves or retires". HM Revenue & Customs. Retrieved 2013-05-18.
- PAYE forms: P45, P60, P11D GOV.UK information
- PAYE forms: P45, P46, P60, P11D Directgov pages in the UK National Archives
- E13 Day to Day Payroll Additional United Kingdom information
- Revenue Commissioners (October 2007). "Employers Guide to PAYE" (PDF). pp. §12.1: Form P45 (cessation certificate) pp.71–3; §12.3: Completing form P45 pp.73–5. Archived from the original (PDF) on 2007-11-19. Retrieved 2007-11-01. Republic of Ireland information
- Revenue Online (2005-07-04). "Document Type Definition P45" (PDF). Archived from the original (PDF) on 2007-11-19. Retrieved 2007-11-01. DTD for online submission of P45 in XML format