The former civil servant, who became a household name after leading the investigation into lockdown-breaking parties in Downing Street, will be allowed to take up the position from September, says the advisory committee on business appointments.

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Sky News revealed in March that Ms Gray had been in secret talks with Labour about the appointment, and within hours she had quit the Civil Service in order to take up the role.

Sky News also reported that conversations between Sir Keir and Ms Gray had begun in October 2022 when she was a permanent secretary working on the constitution and the union of the UK - but after she had published her partygate report.

It is understood that the government plans to make a written statement in the House of Commons on Monday, which will be titled "prima facie breach of the Civil Service code by former second permanent secretary for the union & constitution", which will lay out why the government believes Ms Gray broke the rules by not declaring her contact with Labour.

Prime facie means "at first sight, according to barrister Adam Wagner.

This followed the government's own internal inquiry following an angry reaction from Conservative MPs, who called the situation "unprecedented" and sought to discredit her partygate findings.

Ms Gray's hiring was subject to approval by the advisory committee on business appointments (Acoba), which can impose conditions and a cooling-off period before future employment can begin.

In his letter to Ms Gray, the head of Acoba, Lord Pickles, said: "Ms Gray told the Committee no formal job offer was made until 2 March 2023 and that, until then, no terms had been discussed; nor any likely date for her leaving the Civil Service.

"Ms Gray also said that whilst she had brief personal conversations with the employer during her time in office, no government policy or business was discussed.

"No evidence to the contrary has been provided to the Committee."

Acoba also said 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".

Ms Gray said she was first approached about the job in October last year by Sir Keir Starmer, and spoke several more times between then and March to senior members of the Labour administration.

She told Acoba that terms, government policy, government business, or a leave date were not discussed.

The complete list of requirements Ms Gray must abide by include:

● Waiting six months to take up the role from when she resigned in March;

● No using any privileged information available to her from working for government;

● Not lobbying the government on behalf of the opposition for two years;

● Not discussing government contracts with the opposition for two years;

● Only speak to the government through the Cabinet Secretary.

It is notable that some of the restrictions only apply to Labour's time in opposition, and could lapse if she re-enters government should Labour win the expected election next year.

The six month wait period was decided to avoid "a potential risk to the perceived impartiality of the Civil Servic"e

Sir Keir reacted to the news by saying: "I'm delighted that Sue Gray will be joining Labour as my chief of staff.

"Sue will lead our work preparing for a mission-led Labour government. She brings unrivalled experience on how the machinery of government works and is a woman of great integrity.

"After 13 years of the Tories, we know there is much to do to tackle the cost of living crisis, grow our economy and rebuild our public services.

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"Should we be privileged enough to be elected, Sue will ensure we're able to hit the ground running. I look forward to her starting.

"I'm also grateful to Acboa for their work. We have followed the process and accept their advice."