Keir Starmer is facing furious demands to quit tonight after Downing Street admitted Lord Mandelson was made Britain's ambassador to the US despite failing security checks.

In an astonishing development, No10 confirmed Lord Mandelson was given the go-ahead to take on the role against the recommendation of security vetting officials - but claimed the Prime Minister didn't know.

The Foreign Office is said to have pushed ahead with appointing Lord Mandelson as the UK's top diplomat in Washington DC despite the advice.

Downing Street insisted neither the Prime Minister nor any other Government minister was aware until Tuesday evening, at which point Sir Keir immediately ordered a Whitehall probe.

It only emerged that Lord Mandelson had failed security vetting when The Guardian newspaper published explosive claims on Thursday afternoon - with No10 remaining silent for nearly three hours.

The newspaper reported that security officials initially denied Lord Mandelson clearance, but the PM had already named him as ambassador and the Foreign Office took the rare step of overruling the recommendation.

There were claims at Westminster that Olly Robbins, the architect of Theresa May's Brexit deal who is now the most senior civil servant at the Foreign Office, is being lined up as a fall guy.

The Foreign Office said it is urgently working to comply with Sir Keir's demand to know how Lord Mandelson was cleared to become US ambassador.

The extraordinary revelations have piled more pressure on Sir Keir's position in Downing Street, with the Tories, Reform UK, Liberal Democrats and Green Party all calling on him to resign over his 'lies' about Lord Mandelson's appointment.

It has been pointed out how, in February, the PM publicly insisted that a security vetting process for Lord Mandelson 'gave him clearance for the role'.

Sir Keir has also been accused of misleading Parliament over his previous claims that 'full due process' was followed in the appointment of Lord Mandelson.

Tory leader Kemi Badenoch said: 'The PM appointed Peter Mandelson before the vetting had been completed, vetting Mandelson failed.

'Starmer then said full due process was followed. THAT is misleading Parliament. I'm only holding him to the same standards to which he's held previous PMs - that if they mislead parliament, they should resign.

'In these dangerous times, Britain cannot afford to have a PM who the country doesn't trust. Starmer has betrayed our national security. He should go.'

Reform's Nigel Farage said: 'Now we discover that he has blatantly lied, the Prime Minister should resign.' 

Lib Dem leader Ed Davey also called for the PM's resignation. 'Keir Starmer had already made a catastrophic error of judgement,' he said. 

'Now it looks as though he has also misled Parliament and lied to the British public. If that is the case, he must go.'

No10 sources said information about Lord Mandelson's vetting was obtained by officials trawling through piles of documents, as the Government scrambles to comply with MPs' demand for the publication of all files related to his appointment.

A Government spokesperson said: 'The security vetting process for Peter Mandelson was sponsored by the FCDO.

'The decision to grant Developed Vetting to Peter Mandelson against the recommendation of UK Security Vetting was taken by officials in the FCDO.

'Neither the PM, 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, in order to enact plans to update the House of Commons.

They said the Government is committed to complying with the parliamentary 'humble address' motion to disclose documents relating to Lord Mandelson's appointment 'in full as soon as possible'. 

The cross-party Intelligence and Security Committee (ISC) has been given the final say on what is too sensitive for publication.

'Any documentation within the scope of the humble address that requires redaction on the basis of national security or international relations will be provided to the ISC, the spokesperson added.

'This will include documents provided to the FCDO by UK Security Vetting.'

A Foreign Office spokesperson said: 'The PM has initiated a process to establish the facts of the granting of developed vetting and we are working urgently to comply with that process.'

It is understood that recommendations by UK Security Vetting are non-binding on Government departments.

The PM and Cabinet Secretary are said to have agreed on a 'fact-finding' exercise once the information about Lord Mandelson's vetting was uncovered.

The Government is being forced by MPs to release of a batch of documents about the process after the Commons passed a 'humble address' motion earlier this year.

It is not clear whether the recommendation against Lord Mandelson by vetting officials was because of his ties to paedophile Jeffrey Epstein.

Chancellor Rachel Reeves told reporters in Washington DC this evening: 'I didn't know anything about the vetting process.

'I'm the Chancellor, I'm not the foreign secretary, and I'm not 10 Downing Street, so I can't give you any more information on that.' 

Thursday's developments have revived the furore that already came close to exploding Sir Keir's premiership in February.

One senior Labour source told the Daily Mail that the picture was 'far more complicated' than it appeared.

They said when they learned details of what had happened their initial response was 'holy f***'. 'This is a huge failure,' they added.

Labour MPs have been up in arms that Lord Mandelson was given the key job despite long-standing ties to Epstein.

It is understood Lord Mandelson is adamant that he did not know he had been rejected by vetting officials until the reports today.  

It is also not clear who in the Foreign Office made the apparent decision to overrule UKSV. 

Sir Olly is believed to have been the permanent secretary at the Foreign Office at the time, while Deputy PM David Lammy was Foreign Secretary. 

Senior Labour MP Emily Thornberry, the chair of the House of Commons' Foreign Affairs Committee, said she would summon Sir Olly to clarify information he gave it at a previous hearing.

She said: 'My committee asked several times whether red flags had been raised by Peter Mandelson's vetting process. It seems there were.

'Who overrode these concerns? Why were we kept in the dark? People need to stop messing us about and tell us the truth.'

Developed vetting is standard for most mid-level diplomatic staff. An outright refusal of clearance is believed to be quite rare, although 'mitigations' can be requested.

In another potentially incendiary claim, the Guardian said senior Government officials have been considering whether to withhold documents about the refusal from Parliament. 

Labour MPs rebelled to insist on the publication of a huge range of material about the process, and the cross-party ISC is meant to be having the final say on what is too sensitive for publication. 

The Daily Mail understands no document showing that vetting clearance was initially refused has been supplied to the ISC so far.

Sir Keir previously insisted Lord Mandelson was subject to 'security vetting, carried out independently by the security services, which is an intensive exercise that gave him clearance for the role'. 

Asked about his appointment of Lord Mandelson during a press conference in East Sussex on February 5, Sir Keir said: 'There was a due diligence exercise that culminated in questions being asked because I wanted to know the answer to certain issues.

'That's why those questions were asked. The answer to those questions were not truthful.

'There was then, I should add, security vetting carried out independently by the security services, which is an intensive exercise that gave him clearance for the role, and you have to go through that before you take up the post.

'Clearly, both the due diligence and the security vetting need to be looked at again.

'I've already strengthened the due process. I think we need to look at the security vetting because it now transpires that what was being said was not true. And had I known then, what I know now, I'd never have appointed him in the first place.'

On September 16 last year, Yvette Cooper - by then Foreign Secretary - and Sir Olly wrote to the Foreign Affairs Select Committee in response to questions about the vetting.

'Peter Mandelson's security vetting was conducted to the usual standard set for developed vetting in line with established Cabinet Office policy,' the letter said.

However, it did not mention that UKSV had initially refused clearance. 

Labour MP Bell Ribeiro-Addy said: 'The Government must come clean about why the Foreign Office overrode security officials' decision to deny Mandelson clearance.

'This scandal has been beyond damaging, as has the Government's handling of it. We need the full facts.'

Fellow Labour backbencher Rachael Maskell said: 'All we want is the truth.

'Now a different account of Mandelson's security clearance has been brought to light we need accountability as to why we were provided a different version of events.'

The Prospect union, which represents vetting officers at UKSV, accused No10 of allowing the 'impression to circulate' that people had not done their jobs properly. 

General Secretary Mike Clancy said: 'It is deeply unfortunate that following the resignation of Morgan McSweeney Downing Street allowed the impression to circulate that the vetting of Peter Mandelson had not been done correctly by UK Security Vetting.

'Not only were UKSV put in an invidious position by being asking to conduct vetting after an appointment had been announced, but now deeply troubling reports have appeared in in the media claiming that UKSV advice was overruled.

'Civil Servants, particularly those working in the most sensitive parts of government cannot speak publicly, and deserve ministers to take responsibility for the decisions they take and not to seek to deflect blame onto them.'

Senior Tory MP David Davis suggested Sir Keir must have known if Mandelson had failed his security vetting.

The ex-Cabinet minister posted on X: 'In what sort of Government does the Foreign Office override a vetting failure for the most senior ambassadorship in the world without notifying the Prime Minister?

'It must be presumed that Starmer knew of this vetting failure when he officially appointed him.

'Frankly, this calls into question the Prime Minister's claims that he made the decision because Mandelson had lied to him, since he would have had the accurate conclusions of the vetting process when he made the decision.'

It previously emerged that Sir Keir did not speak to Mandelson personally before appointing him as US ambassador.

The PM is said to have left his aides to ask questions about his ties to Epstein.

That was despite being presented with evidence that Mandelson's friendship with the financier had continued after he was jailed.

National security adviser Jonathan Powell also expressed misgivings during the process.

However, Sir Keir went ahead, after apparently agreeing with chief of staff Morgan McSweeney on three questions that Mandelson needed to be asked.

Mr McSweeney resigned in February saying he took full responsibility for the appointment going ahead. 

Mandelson was arrested on February 23 on suspicion of misconduct in public office, having been accused of passing sensitive information to Epstein during his time as business secretary under Gordon Brown.

He was subsequently bailed, but later handed his passport back and freed under investigation. He has denied any criminal wrongdoing or acting for personal gain.