Former senior official Sir Olly Robbins has accused Downing Street of taking a "dismissive attitude" to vetting during Lord Mandelson's appointment as the UK's ambassador to the US.
Sir Olly was sacked as Foreign Office boss last week after it emerged he had cleared Lord Mandelson to take up the role despite officials raising security concerns, without telling Sir Keir Starmer.
But in front of MPs, Sir Olly defended his actions, insisting he had followed the proper process while under "constant pressure" from No 10 to get Lord Mandelson in post.
No 10 denied claims of a dismissive approach towards vetting, and said it was reasonable to ask for updates on the appointment.
Lord Mandelson was announced as the UK's ambassador to the US in December 2024, with the peer then undergoing in-depth vetting to obtain his required security clearance for the role.
The former Labour minister was then sacked as US ambassador in September last year, after new details emerged about the extent of his friendship with the late convicted child sex offender Jeffrey Epstein. He has since said he regretted ever having known Epstein.
Sir Keir appointed Lord Mandelson to the post, and the decision has dogged him for months, leading to calls for the prime minister to resign.
The controversy was reignited last week, after The Guardian reported Sir Olly's department had gone against a recommendation from vetting officials that Lord Mandelson should not be given security clearance.
In a statement to MPs on Monday, Sir Keir said it was "incredible" he was not informed about the result of the vetting assessment.
But on Tuesday, it was Sir Olly's turn to tell his side of the story in a highly anticipated appearance at the Foreign Affairs Select Committee, making a series of explosive claims in a session that ran for more than two hours.
Sir Olly said his department faced "constant pressure" from Downing Street to formally approve Lord Mandelson's appointment, with a "strong expectation" the former cabinet minister needed to be "in post and in America as quickly as humanly possible".
Having only started in the role two weeks before security clearance was granted, Sir Olly said his predecessor had briefed him that Downing Street felt vetting "might be unnecessary" for someone of Mandelson's status.
"A position taken from the Cabinet Office was that there was no need to vet Mandelson," he told the MPs.
"He was a member of the House of Lords, he was a privy councillor, the risks attending his appointment were well known, and had been made clear to the prime minister before appointment," he added.
"In the end the FCDO insisted and put its foot down, I understand my predecessor had to be very firm in person."
Although he declined to name anyone making these calls, Sir Olly said there was clearly a "dismissive approach" to the process but he was "confident that we were handling it well" and had not "bowed to pressure".
Sir Olly told MPs that the concerns flagged by UK Security Vetting (UKSV), the government agency that did Lord Mandelson's vetting, did not relate to his previous relationship with Epstein, but did not say what they were about.
Sir Olly insisted that the Foreign Office had acted properly when approving the clearance.
He said: "I was briefed that UKSV considered Mandelson a borderline case and that they were leaning towards recommending that clearance be denied, but that the Foreign Office's security department assessed that the risks identified as of highest concern by UKSV could be managed and/or mitigated.
"I was told that UKSV acknowledged, I don't know in what way, but acknowledged that the Foreign Office might wish to grant clearance with appropriate risk management."
Sir Olly also told MPs:
On Monday, Sir Keir denied accusations he had misled MPs when he told them last year that "full due process" had been followed during the appointment.
The government rulebook says ministers who knowingly mislead Parliament are expected to resign, while any inadvertent error should be corrected "at the earliest opportunity".
But the prime minister insisted he would have acted differently had he been informed at the time about the results of the UKSV vetting.
Even so, the Conservatives secured an emergency debate on Tuesday, at which they argued Sir Keir should face a vote of no confidence.
"It is clear to the public that he is failing at the job, it is clear to civil servants that he is throwing them under the bus, and it is clear to members across this House that he is not fit to lead," Tory leader Kemi Badenoch said.
However, only a handful of Labour MPs spoke during the debate to voice discontent with Sir Keir.
One of them was Liverpool West Derby MP Ian Byrne, who called for "a thorough review of the political operation which brought the prime minister to power".
Some MPs raised questions about a claim made by Sir Olly earlier, that Downing Street considered giving the prime minister's then-director of communications Lord Doyle a diplomat role.
In a statement, Lord Doyle said he had never sought such a position, and was not aware of anyone speaking to the Foreign Office about finding a role for him.
No 10 declined to give details about "personnel discussions" - but pointed out Lord Doyle did not take up a role at the Foreign Office after leaving No 10.
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