Sue Gray broke the Civil Service code by discussing a job with Labour leader Sir Keir Starmer, a Cabinet Office investigation has concluded.

In a written ministerial statement, Cabinet Office minister Jeremy Quinn said the “undeclared contact” between her and Sir Keir constituted a breach.

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

But he insisted he remained “confident in the impartiality of our Civil Service”.

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A government source told Sky News that the findings of the investigation – carried out by civil servants, not ministers – would have resulted in “serious disciplinary sanctions” if Ms Gray had still been working in Whitehall.

But a Labour spokesperson said the statement was “a political stunt by a Tory government out of ideas and out of road”.

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Ms Gray became a household name during the partygate revelations, leading the investigation into lockdown-breaking gatherings in Downing Street.

Sky News revealed in March that she had been in secret talks with Labour about becoming Sir Keir Starmer’s chief of staff since October 2022 – when still a senior civil servant, but months after she published her partygate report.

Within hours, she had quit the Civil Service in order to take up the role.

But first, Ms Gray had to get the sign off from the Advisory Committee on Business Appointments (Acoba), which can impose conditions and a cooling-off period before former ministers or senior civil servants start new jobs.

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Sky News political correspondent Tamara Cohen explains the probes into Sue Gray

The watchdog approved her appointment on Friday, saying there had to be a six-month break between her quitting and her starting at Labour to avoid “a potential risk to the perceived impartiality of the Civil Service” – meaning she can begin as chief of staff in September.

However, there was still anger from Conservative critics, who continue to question her impartiality during her inquiry into Downing Street parties.

Acoba said on Friday that “no evidence” was provided to the committee that Ms Gray’s “decision-making or ability to remain impartial was impaired whilst she remained in her Civil Service role”, and that “no evidence has been provided by the departments to demonstrate Ms Gray made decisions or took action in office which favoured the employer in expectation of this role”.

But while Mr Quinn’s statement said the prime minister accepted the advice from the committee, the Cabinet Office inquiry into her discussions with Labour – launched after Sky broke the news about the talks – still criticised her handling of the events.

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Sue Gray’s appointment sparked a fiery debate in the Commons when it was first revealed.

“The Civil Service management code sets out that all members of the senior Civil Service are in the ‘politically restricted’ category, which places further restrictions on their political activity,” he said.

“In addition, the guidance on the declaration and management of interests for civil servants, which is enshrined in departmental HR policies, sets out that individuals must declare all relevant outside interests to their line manager as soon as they arise.”

“This [inquiry] 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.”

Labour insisted “all rules were complied with” and the Acoba process “makes that clear”.

A party spokesperson added: “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.”