You will get a P45 when you stop working for an employer and you need to give this to a new employer.
The P45 shows how much money you have earnt in the current tax year (April-April) and what tax you have paid.
It will show the code that the Tax office has used to calculate your tax and whether they have you on a cumulative or week 1 tax code. The new employer is required to send this to the Tax Office who issue a code detailing how much tax they have calculated you need to pay.
If you don’t have a P45 you will be asked to complete a Starter Checklist (previously called P46).
The Starter Checklist has questions about any other jobs, benefits or student loans you have. It helps your employer work out your correct tax code before your first payday.