Partygate investigator Sue Gray broke civil service rules “as a result of the undeclared contact” between her and the Labour Party, according to a Whitehall investigation.

The former senior civil servant is due to take up the role as Sir Keir Starmer’s chief of staff in September.

She was cleared for the post following a six-month cooling-off period by the Advisory Committee on Business Appointments (Acoba), the anti-corruption watchdog.

However, in a separate Cabinet Office inquiry, it was found that Ms Gray, who refused to give evidence to the Government probe, broke the civil service code due to her contact with Labour ahead of her resignation in March.

Labour branded the investigation “Mickey Mouse nonsense.”

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Cabinet Office minister Jeremy Quin said: “I can now confirm that the Cabinet Office process looking into the circumstances leading up to Ms Gray’s resignation has been concluded.

“As part of the process, Ms Gray was given the opportunity to make representations but chose not to do so.

“This process, led by the Civil Service, found that the Civil Service code was prima facie broken as a result of the undeclared contact between Ms Gray and the Leader of the Opposition.

“The rules and guidance that govern the conduct of civil servants are clear and transparent. It is deeply unfortunate that events have transpired in this way.”

Acoba on Friday recommended that Ms Gray have a six-month cooling-off period, starting from the day she quit the civil service, before taking up her new role with the Opposition, allowing her to start in two months’ time.

The watchdog ruled it had seen “no evidence” that her decision making or impartiality was “impaired” while serving in Whitehall.

Government figures had lobbied Acoba to impose a waiting period of 12 months after complaints about Ms Gray’s move to join the Labour leader after leading a high-profile investigation into Boris Johnson over lockdown breaches in No 10.

But while Acoba chairman and Conservative peer Lord Pickles said he “shared” some of the concerns raised by departments over the potential risk to the civil service’s integrity due to her job switch, he dismissed the call for a lengthier delay.

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Responding to the Whitehall probe conclusion, a Labour Party spokesman said: “All rules were complied with. The Acoba process makes that clear.

“This statement is a political stunt by a Tory government out of ideas and out of road.

“It says everything you need to know about the Tories that they have spent weeks wasting time on this Mickey Mouse nonsense, while refusing to investigate the serious allegations of sexual assault against their London mayoral hopeful Daniel Korski.

“We’re looking forward to Sue Gray joining us this September as we continue to show the country that only Labour can build a better Britain.”

Mr Korski, a former Downing Street adviser during David Cameron’s tenure, pulled out of the race last week to be the Tory candidate to take on Sadiq Khan in the capital after it was alleged he groped a TV producer in No 10 a decade ago.

The Cabinet Office has ruled out investigating the claims, which Mr Korski “categorically” denies, made by Daisy Goodwin.