Sir Keir Starmer has expressed his frustration at being prevented from setting straight the story behind his hiring of partygate investigator Sue Gray as his chief-of-staff while Whitehall’s revolving-doors watchdog reviewed the job offer.

The Labour leader said the Advisory Committee on Business Appointments (Acoba) told him not to share details about his contact with the former mandarin while it considered the suitability of the move.

Acoba had been reviewing whether the appointment risked undermining the Civil Service’s integrity but ultimately found “no evidence” that Ms Gray’s decision-making or impartiality were “impaired” while serving in Whitehall.

I wish the committee hadn't said, 'Don't tell anyone until we've determined'

A separate Cabinet Office investigation earlier this week reached a different conclusion, finding that there had been a “prima facie” breach of the Civil Service Code as a result of “undeclared contact” between the pair.

But Sir Keir said he had “always been confident” that no rules were broken as he set out in more detail his contact with Ms Gray during the run-up to her Whitehall departure.

He told an LBC listener phone-in on Friday: “The committee had asked Sue and me that they should be the people who have this information first and they should be the people to consider it in the proper way.

“Now, that was frustrating for me because it is actually a very short story. I know what the rules are, Sue knows what the rules are, that is why we only had a very brief conversation along those lines.

“I wish the committee hadn’t said, ‘Don’t tell anyone until we’ve determined’. But they’ve got their own process which we respected. And of course they’ve said that nothing was wrong.”

He said his contact with Ms Gray in October 2022 came “months after” her probe into pandemic-era parties in Downing Street under Boris Johnson’s premiership was published.

“The last time I saw Sue Gray before that was at a funeral some years beforehand,” he said.

The conversation last year was a “short call” in which he said “I’m looking for a chief-of-staff, if you were to leave the civil service, is this something you might consider?”, the Labour leader told LBC.

“I didn’t discuss politics, I didn’t discuss policy,” he said.

When Ms Gray’s planned move was later reported in March, he said he called her again “to make sure she was alright and to find out what she was doing”.

“But that was the long and the short of it and that is why I’ve always been confident in saying there was no breach of the code,” Sir Keir told the show.

At the time of her Whitehall departure, Sir Keir had dodged questions on when Labour first approached her with the job offer.

Thrust into the limelight when she took over the probe into coronavirus rule-breaking at No 10 in 2021, Ms Gray went from an influential but little-known arbiter of conduct in Government to a household name within months.

Her move to Labour angered some Tory MPs and a Whitehall inquiry on Monday alleged that the Civil Service Code had been broken by communicating with Sir Keir in advance of her resignation.

Cabinet Office minister Jeremy Quin said: “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.”

Labour branded the probe “Mickey Mouse nonsense” and said the conclusion is a “political stunt by a Tory Government”.

Government figures had urged Acoba to impose a waiting period of a year between Ms Gray’s Whitehall resignation and appointment to Labour, but it advised only a six-month gap, meaning she can start in early September.

Cabinet Secretary Simon Case is set to be quizzed about the former civil servant’s move in an appearance before Parliament’s Public Administration and Constitutional Affairs Committee next week, which is also likely to touch on the propriety of appointments to the House of Lords following Boris Johnson’s controversial resignation honours list.