Mandelson was named to the top diplomatic post in December 2024, just weeks before US President Donald Trump was inaugurated the following month.
British Prime Minister Keir Starmer insisted on Wednesday that he would not resign, claiming allegations of misleading MPs over his appointment of Peter Mandelson as UK envoy to Washington had been "put to bed".
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Starmer's remarks in parliament came as the beleaguered premier faced fresh calls to step down over his admitted error of judgement in appointing the veteran politician to the coveted post.
The premier spoke to lawmakers a day after the foreign ministry's most senior official, Olly Robbins, gave evidence to a parliamentary committee having been fired by Starmer over the affair last week.
Starmer has accused Robbins of failing to tell him about problems that emerged during Mandelson's security clearance.
He has repeatedly insisted that despite previously stating that "all due process" had been followed he would not have allowed the appointment to proceed if he had known that independent vetting officials had recommended security clearance be denied.
Starmer told MPs that Robbins clearly answered "no" when asked if he had shared the recommendation "with me, number 10 or any other ministers".
"That puts to bed all the allegations levelled at me ... in relation to dishonesty," he said, adding a week ago opposition politicians "were all saying that it must have been shared with me ... It was not."
Mandelson was named to the top diplomatic post in December 2024, just weeks before US President Donald Trump was inaugurated the following month. He took up the job in February 2025.
The exact nature of the risks raised by vetting officials has not been made public.
Robbins has said that they did not relate to Mandelson's relationship with late US sex offender and disgraced financier Jeffrey Epstein.
Main opposition Conservative Party leader Kemi Badenoch dismissed Starmer's denial, demanding answers as to why he had not rescinded the appointment when he learned of other controversial issues.
A document produced during the appointment process said "Mandelson remained on the board of the Kremlin-linked defence company, Systema, long after Putin's first invasion of Ukraine in 2014," Badenoch claimed.
"The prime minister told us on Monday that he'd read that due diligence report. Why did the prime minister want to make a man with links to the Kremlin, our ambassador in Washington," she added.
Morgan McSweeney, Starmer's former top aide, who resigned over his role in the crisis, is to appear before MPs on Tuesday.
Robbins said on Tuesday that Starmer's Downing Street office put constant pressure on civil servants to approve Mandelson's appointment and seemed to dismiss security concerns.
Robbins described the tone not as "just please get this done quickly" but "get it done".
"I think, a pretty unmistakable feeling," he said.