Foreign Secretary Yvette Cooper has expressed her "extreme concern" at the delay in informing Labour ministers of concerns raised during Lord Mandelson's vetting for the Washington ambassadorship.
In correspondence with Foreign Affairs Committee chair Dame Emily Thornberry, Ms Cooper requested a thorough examination of evidence previously submitted to ensure complete accuracy.
She wrote in her letter: "I am extremely concerned about information that has now come to light and the fact that it was not previously provided to ministers, which has in turn affected information given to Parliament."
The Foreign Secretary's intervention comes as the scandal surrounding Lord Mandelson's failed security clearance continues to engulf the Government, with questions mounting about who knew what and when.
Sir Keir Starmer will face MPs in the Commons on Monday amid mounting pressure from opposition parties demanding his resignation.
The Prime Minister has described it as "staggering" that he was not informed earlier about Lord Mandelson's failure to pass security vetting, having previously assured Parliament that proper procedures had been observed.
Critics have accused him of misleading the House of Commons with those earlier statements.
In an unusual move on Friday evening, No10 published an official record of the April 15 meeting at which Sir Keir claims he first learned of the vetting problems.
Foreign Secretary Yvette Cooper has expressed serious alarm at the delay in informing Government ministers of concerns raised during Lord Mandelson's vetting for the Washington ambassadorship
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Sir Olly Robbins, dismissed from his position as the Foreign Office's most senior civil servant last week, is anticipated to appear before the Foreign Affairs Committee on Tuesday.
David Lammy, who served as Foreign Secretary when Lord Mandelson was selected for the ambassadorial role, has firmly defended the Prime Minister's position.
Mr Lammy told the Guardian: "I have absolutely no doubt at all, knowing the PM as I do, that had he known that Peter Mandelson had not passed the vetting, he would never, ever have appointed him ambassador."
The Deputy Prime Minister acknowledged that the Foreign Office faced certain "time pressures" to confirm the appointment ahead of Donald Trump's return to the White House.
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Sir Olly Robbins was dismissed from his position as the Foreign Office's most senior civil servant last week
| PANeither Mr Lammy nor his advisers were made aware of the vetting concerns at the time, he has claimed.
Regarding Sir Olly's dismissal, Mr Lammy expressed being "surprised and shocked" by the development, describing him as an "outstanding" civil servant.
Conservative leader Kemi Badenoch has branded Sir Keir "unfit to run the country" in a scathing attack.
"This is not just a political failure. It is a moral one. He has put our national security at risk... he should resign," she wrote in the Mail, accusing the Prime Minister of allowing others to bear the consequences while he remains in office.
The Prime Minister is facing calls to resign following the revelation | PA Liberal Democrat leader Sir Ed Davey has demanded the Privileges Committee investigate whether Sir Keir deliberately misled Parliament.
Reform UK, the SNP and the Green Party have joined calls for the Prime Minister to step down.
Lord Mandelson began his ambassadorial duties in February 2025 but was removed last September following revelations about his connections to convicted sex offender Jeffrey Epstein.
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