Sir Keir Starmer would have blocked the appointment of Lord Mandelson as US ambassador had he known he failed security vetting, Deputy Prime Minister David Lammy has said.

The prime minister has come under pressure after it emerged this week that Mandelson was appointed despite security concerns, with Downing Street saying red flags in the vetting process were not disclosed to them by the Foreign Office.

"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," Lammy told the Guardian.

Starmer is due to answer questions about the scandal in Parliament on Monday.

Opposition parties have called on the prime minister to resign, accusing him of misleading Parliament over his previous statements that due process had been followed in relation to the appointment.

Sir Keir has said he was only informed of the situation on Tuesday, and that it was "staggering" he had not been told sooner that Mandelson failed security vetting, which only began after the former minister was picked to be the UK's representative in Washington.

Lammy, who was foreign secretary at the time of Mandelson's appointment, also said that neither he nor his advisers had been told about the vetting process.

The deputy prime minister also said he was "surprised and shocked" by the departure of Sir Olly Robbins, the Foreign Office's most senior civil servant who was ousted this week over the vetting row.

Lammy pointed out Sir Olly had only been in the job for a few weeks when the vetting report was returned, and told the paper there were "time pressures" on the Foreign Office to have Mandelson in place shortly after Donald Trump's return to the White House.

Lammy's successor Yvette Cooper has previously confirmed Mandelson's vetting was made a "priority clearance", but insisted full checks were still carried out despite the process being fast-tracked.

Dame Emily Thornberry, chair of the Foreign Affairs Committee, has said new revelations have "called into question" evidence Sir Olly gave to MPs in November, during which he did not disclose that the government's security vetting agency advised the Foreign Office to deny Mandelson a high-level security clearance.

On Saturday, Cooper told the committee she had asked for a review of the information given to MPs by officials to ensure it was "fully accurate".

She also said she was "extremely concerned" ministers were not told sooner about issues raised by the vetting process, the details of which have not been publicly disclosed.

Sir Olly is expected to be questioned by the Foreign Affairs Committee again on Tuesday.

His allies have said this week that he was bound by the confidential nature of the intrusive vetting process, hence why it was not disclosed to Downing Street at the time.

BBC News understands Sir Olly has not formally accepted the committee's invitation to give evidence, but friends of his said he was preparing to appear on Tuesday.

The BBC also understands that those close to Mandelson believe the sacking of Sir Olly as permanent secretary is "egregious".

Cooper has also confirmed that Nick Dyer, who has held a number of other senior roles in government, had been asked to run the Foreign Office civil service on an interim basis.

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, 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.

The prime minister is believed to be absolutely furious over the handling of Lord Mandelson's vetting, Chris Mason writes.

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