Keir Starmer was facing mounting calls to quit last night after it emerged that Peter Mandelson was made US ambassador despite failing security vetting.

Foreign Office officials pushed the controversial appointment through regardless, it was revealed, and the Prime Minister faces the deeply damaging allegation that he lied to Parliament.

In a last-ditch attempt by Sir Keir to cling on, Downing Street claimed the Foreign Office had acted alone in granting Mandelson the developed vetting (DV) status needed to take up his job in Washington against the advice of internal security experts.

Insiders yesterday predicted that the most senior civil servant at the Foreign Office, Sir Olly Robbins, would be made the fall guy, and so it proved as he was sacked late last night.

An investigation into what went wrong has been launched by No 10.

Sir Keir has insisted on numerous occasions that disgraced Labour peer Mandelson had been properly cleared to take up Britain's most important diplomatic post.

He was told by opposition leaders last night that he must stand down if he is found to have misled MPs over the scandal, while even some of his own backbenchers admitted his position was untenable.

It was claimed last night that Sir Keir, his ministers and No 10 officials knew nothing about the extraordinary decision to overrule security concerns until earlier this week, when it was discovered as part of the process of sifting through files on the scandal to be published.

But Tory leader Kemi Badenoch said: 'The Prime Minister appointed Peter Mandelson before the vetting had been completed – vetting Mandelson failed.

'Starmer then told me full due process was followed. That is misleading Parliament, that is a resignation offence.'

She went on: 'If Starmer was told Tuesday night, why didn't he take the first opportunity to update the House on Wednesday at midday?

'I'm only holding him to the same standards to which he's held previous prime ministers – that if they mislead Parliament, they should resign. In these dangerous times, Britain cannot afford to have a Prime Minister who the country doesn't trust. Starmer has betrayed our national security. He should go.'

Reform leader Nigel Farage said: 'Keir Starmer said in February that the security services had given Mandelson "clearance for the role". Now we discover that he has blatantly lied, the Prime Minister should resign.'

The Liberal Democrats' Sir Ed Davey added: 'Keir Starmer had already made a catastrophic error of judgment. Now it looks as though he has also misled Parliament and lied to the British public. If that is the case, he must go.'

SNP Westminster leader Stephen Flynn reported Sir Keir to the ministerial sleaze watchdog, saying: 'These new revelations are the most serious yet for the Prime Minister.

'It is vital that we learn when Starmer knew that Mandelson's clearance had been refused, and what role he played in overruling that decision.'

Veteran Labour backbencher Graham Stringer told the Daily Mail: 'If he has lied to the Commons, he has to go now.'

A Tory source said: 'Are Downing Street seriously asking us to believe that Olly Robbins, the most proper of proper civil servants, decided not to tell No 10 or any minister that Mandelson had failed the vetting until this week?'

Conservative peer Lord Kempsell added: 'Having worked in No 10, I find it highly implausible that the Prime Minister didn't get tipped the wink that Mandelson failed his DV. Has he just lied about it from day one?'

Since Mandelson was sacked as US ambassador over his ties to paedophile Jeffrey Epstein, Sir Keir has repeatedly insisted that officials followed the correct procedures but were misled by the New Labour grandee.

As recently as February, the day the Commons passed a motion calling on the Government to publish all the relevant documents in the case, the PM said: 'There was a due diligence exercise, and then there was security vetting by the security services.'

Yesterday's dramatic twist revealed that Mandelson had in fact failed clearance in an exercise carried out by United Kingdom Security Vetting officials in January 2025, weeks after Sir Keir had already announced he would be taking up the role.

It is not known why the agency, which receives top-secret guidance from the security services, raised the red flag.

The concerns were then secretly overruled by the Foreign Office and Mandelson was granted the DV status needed to take up his job the following month.

A Government spokesman said: 'The security vetting process for Peter Mandelson was sponsored by the Foreign Office.

'The decision to grant developed vetting to Peter Mandelson against the recommendation of UK Security Vetting was taken by officials in the Foreign Office.

'Neither the Prime Minister, nor any Government minister, was aware that Peter Mandelson was granted developed vetting against the advice of UK Security Vetting until earlier this week.

'Once the PM was informed, he immediately instructed officials to establish the facts about why the developed vetting was granted.'

There will now be an official investigation into what went wrong.

Despite the sacking of Foreign Office Permanent Under-Secretary Sir Olly last night, he could still face a grilling by MPs on the foreign affairs committee, whose chairman Dame Emily Thornberry said: 'My committee asked several times whether red flags had been raised by Peter Mandelson's vetting process. 

'It seems there were. People need to stop messing us about and tell us the truth.'