The Labour leader expressed frustration at being prevented from setting the story straight while a Whitehall watchdog reviewed the job offer.

He said the Advisory Committee on Business Appointments (Acoba) told him not to share details about the former civil servant 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" Ms Gray's decision-making or impartiality were "impaired" while serving in Whitehall.

However, a separate Cabinet Office investigation earlier this week found there had been a "prima facie" breach of the Civil Service Code as a result of "undeclared contact" between the pair.

In an LBC listener phone-in on Friday, Sir Keir detailed the conversations he had with Ms Gray around her becoming his chief of staff.

"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," he told the show.

"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."

Labour leader 'didn't discuss politics or policy'

He said his contact with Ms Gray in October 2022 came "some months" after her report into pandemic-era parties in Downing Street under Boris Johnson's premiership was published.

The conversation 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.

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When Ms Gray left Whitehall, Sir Keir had dodged questions on when Labour first approached her with a job offer.

When she took over the probe into coronavirus rule-breaking at Number 10 in 2021, Ms Gray went from being an influential but little-known arbiter of conduct in government to a household name within months.