A P45 is a form that is used in the United Kingdom to report an individual's tax details when they leave a job. The P45 form contains information about the individual's pay, tax, and National Insurance contributions, and it is used to help calculate the individual's tax liability for the tax year.

The P45 form consists of four parts: Part 1 is given to the individual when they leave their job; Part 1A is given to the individual's new employer; Part 2 is kept by the individual; and Part 3 is returned to HM Revenue and Customs (HMRC).

When an individual leaves a job, their employer is required to give them a P45 form that contains information about their pay, tax, and National Insurance contributions up to the date they left. The individual should keep Part 2 of the form for their records and should give Parts 1 and 3 to their new employer or to HMRC, as applicable.

If an individual has multiple jobs or is self-employed, they may receive multiple P45 forms during the tax year. It is important to keep all P45 forms for your records, as they can be useful for completing your tax return and for claiming any overpaid tax. If you have any questions about your P45 or need help with your tax return, you can contact HMRC for assistance.