Cracks are appearing in Keir Starmer's Cabinet as he comes under mounting pressure over the Peter Mandelson affair.

In an extraordinary parliamentary hearing on Tuesday, a mandarin sacked by the Prime Minister revealed the 'atmosphere of pressure' created by No10 to approve the New Labour grandee as ambassador to the US, as well as its 'dismissive approach' to security vetting. 

And the scandal took another turn when Sir Olly Robbins revealed that Downing Street had secretly tried to give another plum diplomatic job to Sir Keir's spin doctor.

In a stinging criticism of the PM's judgment, Energy Secretary Ed Miliband publicly admitted he had been against giving Mandelson the prestigious Washington DC posting, well before it was known he had failed his security clearance test.

Asked what he had thought about the appointment at the time, Mr Miliband told Sky News: 'That it could blow up, that it could go wrong. I had a conversation with David Lammy about it before the appointment, and I said I was worried about it. I think he was worried about it too.'

The Energy Secretary also admitted he had 'steered well clear' of Mandelson during his own time as Labour leader.

In another blow to the PM, Foreign Secretary Yvette Cooper told the Commons she was 'extremely concerned' about the revelation that the No10 private office had tried to give Sir Keir's then-director of communications Matthew Doyle an ambassadorial role - and keep it secret from the minister in charge of the department.

Ms Cooper said: 'I am, of course, extremely concerned at any suggestion that the permanent secretary or permanent under-secretary of the Foreign Office would be told not to inform the Foreign Secretary. I can also confirm that the case that he raised... it would not have been an appropriate appointment.'

It was reported on Tuesday that those in Government are privately discussing the situation and are 'looking for Cabinet ministers to take the lead' on showing Sir Keir the door. A source told the Politico news site: 'Cabinet ministers will be judged in the future by their words and actions now.'

A senior Labour MP told HuffPost UK that Sir Olly's evidence had been 'utterly devastating', predicting that support will now move to Greater Manchester mayor Andy Burnham.

Tory leader Kemi Badenoch told a Commons debate on Tuesday: 'The Prime Minister personally decided to appoint a serious known national security risk to our most sensitive diplomatic post. 

'Peter Mandelson was not just a man who had already been sacked twice from government for lying, not just a man who had a public relationship with a convicted paedophile.

'But a man with links to the Kremlin and China. Links so close that they were raised as red flags with the Prime Minister before his appointment.

'This Prime Minister has put the country's national security at risk, and he must take responsibility. It is time for him to go.' Mrs Badenoch added: 'I cannot fathom how the Prime Minister can still claim to have not misled the House on this point.'

Nigel Farage said Sir Keir had been 'fatally weakened' by the latest revelations and predicted he will be ousted after 'devastating' local election results next month.

The Reform UK leader told the Daily Mail he did not believe the PM's account - and said none of it explained his decision to appoint Mandelson in the face of warnings about his character.

'He is fatally weakened... it's only a matter of time,' Mr Farage said.

Five days after his sacking for failing to disclose that Mandelson failed security checks, Sir Olly appeared before the foreign affairs committee at a hastily arranged session on Tuesday. 

He told MPs that there was a 'very, very strong expectation' from No10 that Mandelson 'needed to be in post and in America as quickly as humanly possible'.

And he talked of 'an atmosphere of constant chasing,' with Downing Street asking 'has this been delivered yet?' and a 'pretty unmistakable feeling' of 'get it done'.

Labour grandee Dame Emily Thornberry, chairman of the committee, said she thought Sir Olly had been 'bullied' by No10 into approving Mandelson.

She told the News Agents podcast she thought Sir Keir 'should stay on as Prime Minister', but admitted: 'I think this is trouble. This is very destabilising. It's very worrying that this has happened.'

She was among many Labour MPs who blamed Sir Keir's former right-hand man Morgan McSweeney for pushing Mandelson's appointment and the proposed diplomatic job for Lord Doyle. 

It was claimed the then-Downing Street chief of staff told Sir Olly's predecessor at the Foreign Office - referring to Mandelson's appointment - 'Just f***ing approve it.'

Mr Miliband said Sir Keir should not lose his job, saying: 'I think prime ministers make errors. Prime ministers are fallible. Prime ministers are human.'

In an emergency Commons debate called by the Tories, Labour backbenchers spoke out against what they said was a toxic culture in Downing Street - but blamed it on Mr McSweeney and his think-tank Labour Together, as well as Mandelson.

Imran Hussain said: 'There needs to be a full, transparent and independent inquiry on this whole situation that uncovers the truth and leads to consequences, including for the Prime Minister.'

Responding for the Government, the PM's Chief Secretary, Darren Jones, said Sir Keir 'knows that he should not have made the appointment' of Mandelson. 

'He regrets the decision, and he apologises for it,' he added, and insisted the PM had not misled the Commons.