'This is a resigning matter!' Lib Dem MP calls for Keir Starmer to face parliamentary probe over Peter Mandelson scandal

"We're calling for the Prime Minister to be referred to the Privileges Committee of Parliament so that they can get fully to the bottom of this, see who knew what when," she said.

The Liberal Democrat spokeswoman expressed scepticism that Downing Street was entirely unaware of the situation, stating: "It's very hard to believe that absolutely nobody in No10 knew anything about this."

Ms Smart presented the Prime Minister with a stark choice between two damning conclusions regarding the appointment.

"There are two options - either he knew and didn't tell the truth and decided anyway, or there was such a staggering level of incompetence around him for to enable this to happen," she said. "I think both of those are resigning matters".

The Liberal Democrat spokeswoman argued that Sir Keir had already demonstrated poor judgement before the vetting controversy emerged, pointing to Lord Mandelson's previous dismissals from Government.

"Let's not forget he had been sacked twice from ministerial positions because of poor judgement, poor behaviour, and so it baffled many people that the Prime Minister thought this was a good call to appoint him to begin with," Ms Smart added.

Ms Smart cast doubt on the Government's account that senior civil servant Olly Robbins made the decision independently within the Foreign Office.

"Olly Robbins had been in post for only a couple of weeks at this point," she noted, adding that he was still establishing relationships with his new minister and foreign secretary.

"I find it very hard to believe that he unilaterally made a decision and then went on not to tell the truth repeatedly," Ms Smart said.

She also turned her fire on the Foreign Secretary, declaring: "I think David Lammy also needs to consider his position."

Ms Smart emphasised that under proper governmental procedure, civil servants provide advice while ministers take decisions, whether formally documented or communicated informally.

"I think this story is increasingly incredible," she added.

Ms Smart underscored the gravity of the diplomatic posting at the centre of the controversy.

"His Majesty's Ambassador to the United States of America is one of the most important, sensitive diplomatic roles that we have as a nation, and to appoint somebody who had failed the developed vetting is staggering," she said.

With Parliament not sitting until Monday, Ms Smart suggested the Prime Minister should proactively submit himself to rigorous media questioning.

She said: "I think if I were him and wanting to be on the front foot, I would get the gnarliest, toughest journalist in front of me asking me questions in a till you drop style.

"If he's got nothing to hide, he must surely welcome that level of scrutiny. The buck stops with him."

Breaking his silence on the scandal, Sir Keir said he was "absolutely furious" that the Foreign Office had not made him aware.

He told reporters: "That I wasn't told that Peter Mandelson had failed security vetting when he was appointed is staggering.

"That I wasn't told that it failed security vetting when I was telling Parliament that due process had been followed is unforgivable.

"Not only was I not told, no minister was told. I'm absolutely furious about that. What I intend to do is to go to Parliament on Monday to set out all the relevant facts in true transparency, so Parliament has the full picture."