Sue Gray can start as Keir Starmer’s chief of staff in September, the official appointments watchdog has said.
The former senior civil servant - who conducted an investigation into partygate - quit as an official in March.
Ex-officials and ministers have to ask the the Advisory Committee on Business Appointments (Acoba) before taking on new jobs.
On Friday Acoba recommended Gray be allowed begin her new job with the Labour leader after a six-month delay from when she resigned.
Her appointment by Starmer proved controversial among Tory MPs given that she led the official probe into rule-breaking during lockdown.
Gray’s report in May 2022 proved to be a bombshell. It detailed events at which officials drank so much they were sick, sang karaoke, became involved in altercations and abused security and cleaning staff at a time when millions of people across the country were unable to see friends and family.
She criticised “failures of leadership and judgment” in No 10 and said “the senior leadership at the centre, both political and official, must bear responsibility”.
Six weeks later, Boris Johnson was forced out of office by his own cabinet and Conservative MPs.
Starmer has insisted he had no discussions with Gray while she was investigating Johnson
The purpose of the Acoba rules is to avoid “any suspicion that an appointment might be a reward for past favours”.