United Kingdom Prime Minister Keir Starmer told members of Parliament on Monday that he made a judgment "that was wrong" and for which he "takes responsibility" over the appointment last year of Peter Mandelson as UK ambassador to the United States.

Mandelson quit within months, following revelations about his friendship with the dead sex of fender Jeffrey Epstein, and following his resignation, he was arrested in February and released on bail on charges of misconduct in public office, relating to the alleged sharing of sensitive government information with Epstein.

It has since emerged that Mandelson had earlier failed a vetting process carried out by United Kingdom Security Vetting, but the Foreign Office gave him security clearance anyway, something that Starmer says he was not made aware of until last week. He said, had he been aware of the vetting issue, he would not have allowed Mandelson to be appointed.

Mandelson was forced to resign from two posts under the previous Labour Party government of then-prime minister Tony Blair, which caused many people to question his suitability for such a sensitive diplomatic posting in the first place.

The Labour Party's Scotland Secretary Douglas Alexander told the BBC "a judgment was made that the Trump administration was an unconventional administration, and an unconventional ambassador could do a job for the United Kingdom … that judgment was wrong".

Starmer has strongly denied being aware of what went on, and in parliament he said it was "absolutely unforgivable" that the then-foreign secretary, David Lammy, had been allowed to sign a statement on Mandelson's appointment without being made aware of the full facts.

"Many members across the house will find these facts to be incredible," said Starmer. "To that, I can only say they are right. It beggars belief that throughout the whole timeline of events, officials in the Foreign Office saw it fit to withhold this information from the most senior ministers in our system of government."

The top civil servant in the Foreign Office has since been sacked, but Starmer's political opponents have said the official has been used as a scapegoat, and that the prime minister has been trying to avoid taking responsibility.

Kemi Badenoch, leader of the opposition Conservative Party, said Starmer's own office admitted he had "inadvertently misled" members of parliament, but he had declined taking the opportunity to say so himself. If this is correct, she added, he was obliged "at the earliest opportunity to correct the record". The earliest opportunity came at Prime Minister's Questions last week, and she said he chose not to do so.

"This is a breach of the ministerial code," she went on. "Under that code, he is bound to be as open as possible with parliament."

Ed Davey, leader of the Liberal Democrats, said that by blaming his officials, Starmer "gives every impression of a prime minister in office, but not in power", and that the only way he could help to deliver "a government of honesty, integrity, and accountability" was to resign.