A parliamentary committee looking into the appointment of Peter Mandelson as US ambassador and the depth of his relationship with the convicted child sex offender Jeffrey Epstein will not be afraid to publish material that is embarrassing to the government, its chair has said.

Lord Beamish, who leads parliament’s intelligence and security committee (ISC), said there had to be “maximum transparency” about the vetting process Mandelson went through before he was appointed ambassador to the US in December 2024 and what the government knew about his friendship with Epstein.

The ISC, which has statutory responsibility for oversight of the UK’s intelligence services will assess whether documents should not be released because they compromise national security, rather than jeopardising international relations, the Labour peer told BBC Radio 4’s The World Tonight.

Beamish said it was possible the committee could publish documents that impacted international relations. “In the past, for example, we’ve had disputes where things have potentially been embarrassing to governments, but we’ve put them in because it doesn’t jeopardise national security,” he said.

Asked if he felt Starmer had let the Labour party down, Beamish said the prime minister was a “decent individual” but the furore over the release of documents about Mandelson’s appointment had been badly handled. “We do need to ensure that we get maximum transparency,” he said. “If the committee which I chair, can then also get as much transparency as possible around the information which can’t be put in the public domain, that’s … what we need to do.”

On Wednesday Downing Street had attempted to mitigate the Conservatives’ attempt to trigger the release of the documents by adding exemptions for national security and to protect international relations. But MPs called the move a “cover-up” and demanded that judgment on their release be taken by the intelligence and security committee rather than the cabinet secretary. Whips were forced to draft a second amendment to appease MPs, which was passed on Wednesday night. No 10 said it would comply as soon as possible, in accordance with police advice.

It had said it hoped to release documents on Wednesday, but was prevented from doing so by the Metropolitan police, which said certain records could not be released in case they prejudiced a criminal investigation into Mandelson’s apparent sharing of confidential government documents with Epstein.

Tempers continued to flare at Westminster on Thursday after a day of anger among Labour MPs, who warned Starmer’s days as prime minister were numbered after he confirmed publicly that he was aware of reports that Mandelson had continued his association with Epstein after the latter’s conviction.

Further revelations in the Epstein files suggest Mandelson offered to help Epstein obtain a Russian visa, which the disgraced financier planned to use to meet young women in Moscow, BBC News has reported. There is no indication that the politician knew why Epstein wanted the visa, and later emails suggest the trip was cancelled because it could not be obtained, it said.

On Thursday the housing secretary, Steve Reed, said Mandelson had “conned everybody”, and that the prime minister and his chief of staff, Morgan McSweeney, were safe in their jobs.

“The person at fault here is not the prime minister or his team,” he told Sky News. “It is Peter Mandelson who lied, manipulated and deceived everybody, including the media … He conned everybody.”

Labour MPs continued to voice disappointment about the handling of the scandal. Paula Barker, the MP for Liverpool Wavertree, told Radio 4’s Today programme Starmer had broken every pledge he had made when he stood to become leader of the Labour party and questioned his judgment.

Asked how she felt when Starmer confirmed he knew about reports of links between Epstein and Mandelson before his appointment as ambassador, she said: “I was disappointed. I was sickened. And quite frankly, I think the country deserves better.”

Barker stopped short of calling on Starmer to resign, but said: “I think the prime minister has shown that his judgment is questionable. I think he has questions to answer … I think he has a very long way to go to build trust and confidence with the public and trust, and confidence within our party as a whole.”