Keir Starmer would have blocked Peter Mandelson from becoming Britain's top diplomat in a highly embarrassing public U-turn if he knew he failed his security vetting, senior ministers claimed today. 

They leapt to the defence of the beleaguered Prime Minister as he faced ever-increasing questions and pressure over the revelations that experts who deemed Lord Mandelson a security risk were over-ruled.

Deputy PM David Lammy and Work and Pensions Secretary Liz Kendall both went out to bat for No10 today ahead of a week in Westminster that could define - and possibly end - Sir Keir's time in power.

Ms Kendall insisted that the PM was 'a man of integrity' who would have acted if he had been told at the time in January 2025, despite having already publicly announced he was making the former New Labour minister the UK's man in Washington.

And in an interview with the Guardian, Mr Lammy said: '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.'

However, their claims were questioned by critics, with Tory shadow minister Alex Burghart telling Sky's Sunday Morning with Trevor Phillips: 'The truth is, the Prime Minister wants to blame everybody but himself. This was his appointment and he's got to take responsibility for it.

'I find it very difficult to believe that nobody took the Prime Minister aside and said, ''so, you know, sir, there are some very serious concerns here''.'

Appearing on the same programme, the former SNP MP Joanna Cherry said: 'The bottom line is that the PM had already appointed Mandelson, the appointment had been announced, he had been advised against appointing Mandelson by a number of different people and sources.

'Does anyone seriously believe that if he had this information about the vetting he would have changed his mind? It is kind of a red herring in my opinion.'

In a torrid interview on Sky News this morning, Ms Kendall continued Labour attempts to pin all the blame for Mandelson on Sir Olly Robbins, the senior civil servant fired on Friday. 

She accused the former Foreign Office permanent secretary of 'a failure of judgement' amid claims he knew Mandelson's vetting raised serious issues but allowed him to pass anyway.

Speaking on Sunday Morning with Trevor Phillip she said that Sir Keir had done more for violence against women and girls than any other modern PM.

But she failed to then explain how this tallied with him making a known close associate of billionaire paedophile Jeffrey Epstein, a well-documented abuser of women and girls, ambassador to the US in the first place.

Sir Keir is facing mounting calls to resign today ahead of a showdown in the House of Commons on the issue tomorrow, with Tory leader Kemi Badenoch telling the Mail on Sunday today he was 'taking the public for fools' and was unfit to govern.

Mandelson was publicly appointed to the post in December 2024, but red flags came up on his subsequent vetting in January. 

Any U-turn at that stage would have been very embarrassing for the Government, but asked what Sir Keir would have done if he had known about the vetting failures if he had known, Ms Kendall said: 'He would have stopped that.'

Mandelson was granted 'developed vetting' approval against the advice of UK Security Vetting, which conducts it for the government, it emerged last week.

DV is the highest level of security clearance for people required to have 'frequent and uncontrolled access to top secret assets or require any access to top secret codeword material'.

Sir Keir had previously stated that Mandelson passed security vetting, and told the Commons that 'due process' was followed. 

It comes ahead of a crucial week which could decide the Prime Minister's future, starting with his statement to MPs in the Commons tomorrow in which he will reiterate claims he did not know about Mandelson's vetting status.

In a desperate response to the revelations, Sir Keir will also be expected to justify sacking Sir Olly Robbins – the most senior Foreign Office civil servant – over his department's decision to overrule the security verdict.

But just 24 hours after tomorrow's Commons showdown, Sir Olly is expected to publicly defend himself for the first time at an explosive meeting of the foreign affairs committee.