Vetting of government employees and appointees is carried out by an agency overseen by the Cabinet Office, which reports back to hiring departments with recommendations.
BBC News understands the vetting report returned a “no” verdict on whether a security clearance granting access to sensitive government material should be given to Lord Mandelson.
He formally took up the ambassador post in February 2025, before being removed last September when further details about his ties with convicted sex offender Jeffrey Epstein came to light.
It has emerged that Cabinet Secretary Antonia Romeo found out about Mandelson’s vetting failure in late March, but had to investigate the circumstances and legal implications before informing the prime minister.
The Cabinet Office said senior officials “did the right thing” and had taken “urgent steps” to update the prime minister.
Writing in the Mail on Sunday, external, Conservative leader Kemi Badenoch accused Sir Keir of letting others take the fall while he clings to power and branded him as “unfit to run the country”.
“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.
Liberal Democrats leader Sir Ed Davey called for an investigation by the Privileges Committee into whether the prime minister intentionally misled Parliament.
The SNP, Green Party and Reform UK have also called for Sir Keir to resign.