Once you start reporting your PAYE information in real time you won't be able to submit forms P45 via the HM Revenue and Customs (HMRC) PAYE for Employers online service - this is because you must tell HMRC about people who've started working for you, or left, using your real time submissions (Full Payment Submissions, or FPS) through your payroll software.
When someone leaves your employment you must include the date of leaving on the Full Payment Submission (FPS) when you make their final payment. You must also give the employee a form P45. The employee keeps Part 1A for their records.
When an employee starts a new job they will give you the P45 parts 2 and 3. You must record all the information from the P45 on your payroll to make sure you are operating PAYE correctly for them and send details to HMRC when you make your first payment to them. Send your pay and employment details, including the start date, to HMRC on an FPS when you make your first payment to them.