Sir Keir Starmer's claim that he could not tell MPs about the Peter Mandelson vetting fiasco until today has been undermined by the revelation that his top aides have known about it for weeks.

The Prime Minister faces accusations of breaking the Ministerial Code by not immediately telling the Commons that new evidence had cast doubt on his previous assurances that 'full due process' had been followed in appointment of the US ambassador.

Sir Keir was told about it last Tuesday evening, meaning that he could have made a statement to MPs before Prime Minister's Questions on Wednesday or at any point on Thursday when Parliament was sitting.

His allies have insisted that he had to gather the full facts first.

But it has since emerged that two of the most senior figures in Whitehall had in fact been secretly informed about the vetting failure weeks earlier and were already in full possession of the facts and had passed this on to Sir Keir.

Cat Little, Permanent Secretary at the Cabinet Office, had been given a document revealing that officials at UK Security Vetting, the government body responsible, advised against granting Mandelson 'developed vetting' clearance at the end of March.

She received it because she is in charge of the so-called 'humble address' process of gathering files on the Mandelson affair after MPs voted for them to be made public.

Ms Little then discussed the new evidence with the country's top civil servant, Cabinet Secretary Dame Antonia Romeo, and went to government lawyers with her findings before the PM was finally told last week.

Last night, senior Tory MP Alex Burghart said this proved that Sir Keir could have come to the Commons far sooner.

He told the Daily Mail: 'Senior people in the heart of government had been investigating this issue for weeks. 

'On Tuesday, the Prime Minister knew enough to know that he had misled Parliament. And yet he did nothing. 

'It was only when journalists broke the story that the truth started to come out.

'The Prime Minister has broken the rules of the House. He should take responsibility.'

The Lib Dems called for the Commons Privileges Committee to decide whether the PM misled Parliament, and has urged the parliamentary ethics adviser to see whether he had breached the Ministerial Code.

The party's Cabinet Office spokesman, Lisa Smart, said: 'This is yet another clear-cut example that the Prime Minister has shown catastrophically poor judgment and endangered national security.

'It is outrageous that this information keeps being drip-fed to the public, rather than the Prime Minister being honest. 

'It completely undermines and betrays the public's trust. He must go.'

However, a Cabinet minister said yesterday that Sir Keir – who visited the Faslane nuclear submarine base in Scotland on Saturday – had to get all the information necessary before he addressed MPs.

Science and Technology Secretary Liz Kendall told the BBC: 'I think one thing we've learnt from this whole torrid episode is the need to get the facts absolutely clear and right.'

A Labour MP also insisted yesterday that the PM needed time before he addressed Parliament.

Backbencher Tom Hayes told GB News: 'He instructed his Cabinet Secretary to assemble the facts... he's then going into Parliament at the earliest opportunity – Parliament doesn't sit on Friday, and on Monday he'll be coming to the Commons.'

Told that the Prime Minister already had the facts, Mr Hayes replied: 'He had the facts given to him at that point, and he wanted to road-test them, assemble all the facts and make sure the Commons has all the information that is accurate and it needs to know on Monday.'