Your employer is obliged to provide you with a P45 when you leave employment. If you have mislaid this, and are therefore unable to provide a copy to your new employer, you may find you have to pay more tax initially in your new job until your correct code number is confirmed by HM Revenue and Customs.
This is because your employer may need to use an emergency tax code. See the Low Incomes Tax Reform Group website for more information on emergency coding.
Furthermore, any repayment of tax you may be entitled to at the end of claiming Jobseeker’s Allowance (JSA) will not be made automatically by the Jobcentre, unless you have provided a P45. Without your P45, you will need to make a repayment claim directly to HMRC instead.
If you haven’t got a P45 to give to your new employer, ask your new employer for a starter checklist (which replaced the P46 form) to complete. The starter checklist will help your new employer establish the tax code to use against your first salary payment. Your employer will send the form to HMRC once they have processed the information you provided.
Depending on the information you provide, your code may not require further amendment. If a change is necessary, HMRC will advise your employer of your correct tax code once they have obtained information regarding any previous employment or JSA claims you may have made.
Any tax over-deducted by your employer before the correct code is issued by HMRC will be refunded through your wages as soon as your new code is used.