SolvedCSLM10164 | FAQs: employers questions and answers: stop notice received - employee has already left - P45 not yet issued
From HM Revenue & Customs · Collection of Student Loans Manual
Suggested Answer
If the borrower is no longer employed by you and you have not submitted leaver details to HMRC:
leave the box headed "Enter ‘Y’ if student loan deduction is to be made" blank
submit leaver information in Real Time Information and provide employee with P45 parts 1A, 2 and 3.
More information on stop notices can be found on GOV.UK.