Sir Keir Starmer's aides in No10 rejected two offers from the Cabinet Office's ethics chief to question Lord Mandelson over his links to the convicted paedophile Jeffrey Epstein.
A due diligence report, which was compiled by the Cabinet Office's propriety and ethics team in December 2024, warned that Lord Mandelson's association friendship posed a major "reputational risk".
Darren Tierney, who was leading the Cabinet Office's ethics unit at the time, is believed to have approached the Prime Minister's main allies in late 2024, The Telegraph has revealed.
The top mandarin first made contact with Morgan McSweeney's private secretary before his team completed its dossier on Lord Mandelson.
The Lord Mandelson scandal is posing a huge challenge to Sir Keir Starmer's premiership
|PA
Mr Tierney was said to have made a similar offer once the report, which found Lord Mandelson had stayed in Epstein’s home while he was in prison for that offence, was completed.
Mr McSweeney, who served as Sir Keir's chief of staff before resigning over the scandal last month, was in charge of clearing Lord Mandelson's appointment.
Sir Keir has repeatedly claimed he would "never have appointed" Lord Mandelson if he had known about the "depth" of the Labour peer's friendship with Epstein.
He even accused the New Labour architect of "lying" about the extent of his connection to the convicted paedophile.
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Meanwhile, Sir Keir has been accused of swerving questions about whether he spoke to Lord Mandelson about his relationship with Epstein before picking him as Britain's man in Washington.
During a fiery exchange at Prime Minister's Questions, Tory leader Kemi Badenoch said: "The Prime Minister tried to avoid scrutiny on the Mandelson files by releasing the documents immediately after Prime Minister's Questions last week.
"So let me ask him now, did the prime minister personally speak to Peter Mandelson about his relationship with the convicted paedophile Jeffrey Epstein before appointing him as our ambassador to Washington?"
Sir Keir replied: "This was my mistake in making the appointment, and I've apologised to the victims of Epstein, I do so again.
Keir Starmer was warned of Lord Mandelson’s Epstein links just months after entering Downing Street | GOVERNMENT "The matter of process was looked at by the independent adviser on ministerial standards. It's clear the appointment process wasn't strong enough, and that's why I've already strengthened it.
"But it was my mistake, and I've apologised for it. She should follow suit and apologise for her gross error of judgement in calling for the UK to join the war in Iran without thinking through the consequences."
Mr McSweeney was said to have asked Lord Mandelson questions about his friendship with Epstein over email, including on his contact with the convicted paedophile after his conviction.
Lord Mandelson's responses were later approved by ex-No10 communications chief Lord Doyle.
Andrew Mountbatten-Windsor and Lord Peter Mandelson are seen wearing bathrobes seated alongside Jeffrey Epstein | REUTERSLord Doyle, who was a close ally of Lord Mandelson, was later stripped of the Labour whip over his friendship with a paedophile councillor.
Downing Street insisted interviews were not mandatory in the vetting process and disputed claims that Mr Tierney offered to sit down with Lord Mandelson for a second time.
A Government spokesman said: “As set out by the Independent Adviser on Ministerial Standards, the process in place at the time for a political appointee was followed.
“The Prime Minister has taken steps to strengthen the process for political appointments going forward.”