Boris Johnson allies furious as partygate investigator Sue Gray plans to join Sir Keir Starmer's office

The Labour leader was “delighted” that the senior civil servant plans to “accept the role subject to the normal procedures”, his party said.

Ms Gray, who ran a pub in Newry in the 1980s with her husband Bill Conlon, a country and western singer from Co Down, had been one of the shortlisted candidates to succeed David Sterling when he stepped down as head of the Northern Ireland Civil Service in August 2020.

The circumstances of her resignation as second permanent secretary in the Department for Levelling Up, Housing and Communities – accepted with “immediate effect” yesterday – will be reviewed.

Ms Gray's investigation into law-breaking parties held in Downing Street during Mr Johnson's leadership played a key role in his downfall as prime minister.

A Labour Party spokesman said: “The Labour Party has offered Sue Gray the role of chief of staff to the Leader of the Opposition.

“We understand she hopes to accept the role subject to the normal procedures. Keir Starmer is delighted she is hoping to join our preparations for government and our mission to build a better Britain.”

Under the civil service code, officials of Ms Gray's seniority must wait a minimum of three months before taking up outside employment.

The move will be scrutinised by Parliament's anti-corruption watchdog, the Advisory Committee on Business Appointments (Acoba), before advising the prime minister on whether the move is “unsuitable”.

Rishi Sunak will make the final ruling over the rules. Acoba does not have the power to block an appointment.

Arch Johnson loyalists were outraged after Sky News first reported the move, with Jacob Rees-Mogg saying: “So much for an impartial civil service, the Gray report now looks like a left-wing stitch up against a Tory prime minister.”

Nadine Dorries, who served as Mr Johnson's culture secretary, said the reported move to Sir Keir's office was “not surprising”.