Allies of Boris Johnson have reacted furiously as partygate investigator Sue Gray plans to join Labour leader Sir Keir Starmer’s office as his chief of staff.

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

The circumstances of her resignation as second permanent secretary in the Department for Levelling Up, Housing and Communities – accepted with “immediate effect” on Thursday – 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, with Jacob Rees-Mogg saying her appointment “stinks” and calling for an inquiry into her contacts with Labour.

Speaking on GB News, he said: “It is hard not to feel that she has been rewarded and offered a plum job for effectively destroying a prime minister and creating a coup.

“This appointment invalidates her partygate report and shows that there was a socialist cabal of Boris haters, who were delighted to remove him.”

Nadine Dorries, who served as Mr Johnson’s culture secretary, described the Gray report as a “stitch up” and said the reported move to Sir Keir’s office was “not surprising”.

Bassetlaw MP Brendan Clarke-Smith said he was “genuinely shocked”, and accused Sir Keir of having “scant regard for the public image of the civil service and the damage this will do”.

“After the events of last year, people will quite understandably be questioning the appropriateness of this appointment, including issues of impartiality,” the MP added.

Mr Johnson received one of the 126 fines issued by the Metropolitan Police while investigating parties in Downing Street and Whitehall that breached lockdown laws.

One well-placed friend of Mr Johnson claimed the validity and findings of Ms Gray’s partygate report had been “completely destroyed”.

“Keir Starmer appointing Sue Gray as his chief of staff reveals what many have suspected all along: partygate was a deliberate and manufactured plot to oust a Brexit-backing Conservative prime minister,” the ally said.

Whitehall officials were stunned by the “pretty unprecedented” move, with one reacting to reports by saying: “Oh Christ.”

The Institute for Government think tank’s Alex Thomas said the move would be “difficult for the civil service”, saying it would give “critics a stick”.

“Tricky development for those defending impartiality,” he added.

A Cabinet Office spokesman said: “We are reviewing the circumstances under which she resigned.”

Ms Gray got the job as partygate enforcer after Cabinet Secretary Simon Case had to step down from the investigation when he became embroiled in claims of partying during the pandemic.

Published in May last year, her investigation detailed how officials drank so much they were sick, sang karaoke, became involved in altercations and abused security and cleaning staff.

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, Mr Johnson was forced out of office by his own cabinet and Conservative MPs.

On Thursday a No 10 spokesman said: “I can confirm Sue Gray has resigned from the post of second permanent secretary in the Department for Levelling Up, Housing and Communities.

“This was accepted by the department’s permanent secretary and the Cabinet Secretary with immediate effect.

“We will not be commenting further on individual personal matters. But you asked me specifically when, that took place this afternoon.”

Asked whether there were concerns about unpublished partygate details being passed to the Opposition, the spokesman said: “All civil servants leaving the civil service have to abide by the Acoba process.”