Two civil servants who first discovered that Peter Mandelson had failed security vetting ahead of his appointment as US ambassador have denied they left Prime Minister Sir Keir Starmer in the dark.

It was revealed last night that the country's top civil servant, Dame Antonia Romeo, and the Cabinet Office's Cat Little found out last month that Mandelson was given the green light for the role against the advice of security officials.

The Prime Minister has insisted he, and no other minister, had no idea Mandelson - who was forced out of his role in the US as a result of his links to paedophile Jeffrey Epstein - had failed vetting until he was informed on Tuesday. 

He said he was 'absolutely furious' that the Foreign Office had not told him it had overruled the vetting, and sacked its most senior figure, Sir Olly Robbins.

But the revelation that two senior civil servants found out weeks ago, and failed to inform Sir Keir, has raised questions over his control over his government.

On Saturday, a Cabinet Office spokesperson hit back at the accusations, saying Dame Antonia and Ms Little did not share the information sooner because they were waiting for legal checks on what they could release.

They said: 'Senior officials in the Cabinet Office did the right thing and took urgent steps to ensure they could update the prime minister.

'On receipt of the UK security vetting information, the permanent secretary of the Cabinet Office rapidly sought legal and other advice to be in a sound position to share the document, or the fact of it, with the prime minister. She updated the cabinet secretary who agreed with the approach.

'As soon as these checks were conducted, the prime minister was informed.'

It was reported yesterday that Ms Little, the Cabinet Office's Permanent Secretary who is overseeing the publication of files on Mandelson's appointment, was first handed a document highlighting the failure at the end of March.

Written in early 2025 by UK Security Vetting officials, it identified highly sensitive concerns about Mandelson and concluded he should not be given the clearance needed for the job. 

She told Cabinet Secretary Dame Antonia, who was little more than a month into the job, and they discussed the potential risks of sharing the information. 

The Cabinet Office then got legal advice on whether it could prejudice the Metropolitan Police's criminal investigation into Mandelson and also asked the Foreign Office why it had granted him developed vetting clearance against advice.

It was suggested up to a dozen officials and lawyers were aware of the vetting failure, but not the PM, who repeatedly insisted everything was done by the book.

The pair finally approached Sir Keir on Tuesday with details of the revelation that emerged publicly on Thursday.

The Cabinet Office denied that this meant the senior civil servant had sat on the details, while a Government source said: 'Cat has been the one doggedly fighting Olly Robbins to get the documents out of his clutches and into the public domain.

'This is a laughable attempt to shift blame from the actual person who kept the PM in the dark.'

The Foreign Office's chief civil servant Sir Olly Robbins was sacked on Thursday as he had lost the confidence of Sir Keir and Foreign Secretary Yvette Cooper.

The leaders of the major opposition parties have all called for Sir Keir to resign over the latest revelations, with Kemi Badenoch claiming the Prime Minister had misled Parliament and the public.

He is facing a rocky few days as he bids to save his premiership, as he is due to address MPs on the issue in Parliament on Monday.

Sir Olly will then appear on Tuesday to explain his department's role in Lord Mandelson's failed security clearance.

Sir Olly's predecessor at the Foreign Office publicly backed him today following his sacking on Thursday saying he was simply 'following the rules' and accused the Prime Minister of 'throwing him under the bus'.

Criticising the PM as 'wrong', Lord Simon McDonald accused Number 10 of 'wanting a scalp as quickly as possible within the news cycle' and not even giving Sir Olly, who he says was following legal process, the chance to give his own side of the story.

He suggested that the decision to appoint Mandelson had already been made and that the Prime Minister 'wanted his man' in Washington and it was 'an interpretation' that the Foreign Office was effectively left to make it work.

He also warned in stark terms that the Foreign Office was now facing 'its biggest crisis' in more than four decades and needed a new Head 'as quickly as possible'.

Lord McDonald, permanent secretary at the Foreign Office from 2015 to 2020, told BBC Radio 4's Today programme that Number 10 'wanted a scalp and they wanted it quickly'.

He added: 'I think this is the biggest crisis in the diplomatic service since I joined it in 1982.'

Lord Mandelson, a political appointment rather than a career diplomat, was sacked from his Washington role last September when more details emerged about his relationship with paedophile financier Jeffrey Epstein, who died in 2019.