Police officers could quiz Peter Mandelson as part of their criminal investigation within days, it is understood.
The former Labour peer, 72, is being probed by the Metropolitan Police over emails that allegedly show him leaking confidential government information to convicted paedophile Jeffrey Epstein.
Officers are looking at the relationship between the two men during the 2008 financial crisis, and have examined vetting documents related to his appointment as British ambassador to the US in 2024.
Downing Street had planned to publish the vetting documents on Wednesday to try to exonerate Prime Minister Sir Keir Starmer and show that Lord Mandelson lied to him about his friendship with Epstein.
But the Met asked that No 10 delay releasing the files in a bid to avoid prejudicing its investigation.
Government officials have concerns that the probe could go on for weeks, putting further pressure on Sir Keir and Morgan McSweeney, his embattled chief of staff.
A Labour MP said Sir Keir looked “totally out of touch with how the public are feeling” and predicted his claim that he had no reason not to believe Lord Mandelson would “come back to haunt him”.
Speaking to the Press Association anonymously, the MP said: “The Prime Minister pressing on with that event today as if it were just business as usual made him seem totally out of touch with how the public are feeling.
“And his comments to the press that he had no reason not to trust Mandelson have just made things worse for him and will come back to haunt him.”
It comes as Parliament’s Intelligence and Security Committee (ISC) said it could not commit to a timetable for reviewing documents relating to the vetting Lord Mandelson underwent to become ambassador to the US.
In a letter to the Prime Minister, the committee said it would act “as it always does, entirely independently of Government” in determining whether certain documents should be withheld for national security reasons.
The ISC said: “That must be a matter for the committee alone – and it is clearly not possible for the committee to determine this until it sees the papers, or indeed to commit to any timetable until we know the size of the task at hand.”
Asking the Government to provide a date on which it would send the material to the committee, the letter said ministers would have to provide “a clear and logical rationale” as to why publication of any documents would be “prejudicial to UK national security or international relations”.
