A p45 is document issued by your UK employer when you stop working for them and it shows how much tax you’ve paid on your salary/wages so far in the tax year (6 April to 5 April). The p45 has four parts: Part 1 (which is sent by your employer to HMRC), Part 1A (which you keep for your own records) and Parts 2 and 3 (which you hand to a new UK employer, when applicable).