Sir Keir has promised to release files, which he says will prove Lord Mandelson lied about the extent of his friendship with Epstein when he was being vetted for the US ambassador role.
The government was forced into a climbdown by Labour backbenchers, after it had planned to withhold some sensitive material.
After a Commons vote, the government will pass documents which could damage national security or diplomatic relations to Parliament’s Intelligence and Security Committee (ISC), rather than withholding them.
The ISC says the government should decide which documents are made public – and which are not.
They say this will be done by the cabinet secretary, with an expectation the documents are laid before parliament “very shortly”.
At the time Lord Mandelson was appointed as US ambassador in December 2024, it was public knowledge that he had maintained a friendship with Epstein after his conviction for soliciting sex from a minor.
In 2023, the Financial Times reported emails suggesting he stayed at Epstein’s Manhattan mansion in 2009, while the financier was in jail.
The PM said Lord Mandelson was asked directly about the nature of his relationship with Epstein before he was appointed as ambassador, whether he had stayed at his home after his conviction and whether he had accepted gifts and hospitality from him.
“The information now available makes clear that the answers he gave were lies,” he said.
“He portrayed Epstein as someone he barely knew.
“And when that became clear and it was not true, I sacked him.”
The BBC understands Lord Mandelson’s view is that he answered questions about his relationship with Epstein in the vetting process accurately.