Sir Keir Starmer has been handed a stark warning by a former Attorney General following Sir Olly Robbins's "explosive" testimony to the Foreign Affairs Committee.
Speaking to GB News, Sir Michael Ellis said the Prime Minister is "teetering on the edge" and is at risk of "becoming a museum piece".
During his questioning on Tuesday morning, Sir Olly admitted there was a "strong expectation" and "constant pressure" from No10 regarding Lord Mandelson's appointment as US Ambassador.
Sir Olly told MPs that it would have been "very difficult" to have denied the disgraced peer his security clearance, despite the United Kingdom Security Vetting (UKSV) "leaning against" giving him the green light.
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Reacting to the grilling, Sir Michael told GB News that the Prime Minister is in "real trouble" as it appears the appointment of Lord Mandelson was a "rush job".
Sir Michael said: "The Prime Minister is at the British Museum this morning, and will he become a museum piece himself is what I'm thinking, because this was an extraordinary testimony from Olly Robbins.
"The Prime Minister, I think is in real trouble. It was a devastating testimony, and he's going to be under considerable pressure in the House of Commons and not just from the opposition benches, I think also from some on his own back benches.
"So the Prime Minister, I think his teetering here, this was explosive evidence, and as I say, the PM could become a museum piece himself if he's not very careful."
Sir Michael Ellis has warned that Keir Starmer is 'teetering on the edge' following Olly Robbins's 'explosive' testimony
|GB NEWS / PA
The former Attorney General turned to the remarks made by Sir Olly about the "pressure" he faced from Downing Street, and how it has been suggested that Lord Mandelson's appointment was already a "done deal" before vetting began.
He recalled: "I think this is teetering on the edge. If we look at what Olly Robbins was saying, he said that his department was under constant pressure to approve Mandelson.
"Effectively the whole essence of what he was being asked to do was just get it done, so there was constant pressure. He said that the No10 machine appeared to take what he called a dismissive attitude to vetting - the entire focus was on just getting Mandelson to be the ambassador in the District of Columbia just as soon as possible.
Sir Michael continued: "Mandelson's appointment was approved before the inauguration, it was a rush job. There was a public announcement made, His Majesty's approval was sought by No10, they did all that they could do to present a fait accompli to the vetting process.
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Sir Olly Robbins delivered his damning evidence to the Foreign Affairs Committee this morning | PARLIAMENT TV"It seems to me, and Olly Robbins alluded to the fact that therefore it was almost positioned as a done deal.
"He actually said it would have been very difficult indeed to block Mandelson's appointment as he was basically appointed before the vetting was complete, and that seems to me to be because of the prime minister's pressure."
Criticising the Prime Minister for laying blame on the Foreign Office during his statement on Monday, Sir Michael told GB News that the "buck stops everywhere else" but Sir Keir's desk.
He said: "I think it was President Truman who had a sign on his desk saying 'the buck stops here'. Well, that doesn't seem to be on the prime minister's desk, because it seems that the buck stops everywhere else.
Sir Michael told GB News that Keir Starmer risks becoming a 'museum piece'
|GB NEWS
"Olly Robbins was asked if he regretted anything, and he said, I regret vetting was not done before the announcement. I regret the serious reputational risks that the Prime Minister ignored, but I have no regrets about my brilliant team.
"So I think that's an illusion there that Olly Robbins is giving that he actually is criticising the judgement of the Prime Minister in making the appointment in the manner and style that he did, and in the face of the evidence that he had. So I do think this is a serious issue for No10."
During his testimony, Sir Olly Robbins admitted there was a "dismissive approach" by No10 towards the vetting of Lord Peter Mandelson, which meant there was a "very, very strong expectation" that he would become the UK's chief diplomat to the US.
Speaking to the Committee, Sir Olly said: "I'm afraid I walked into a situation in which there was already a very, very strong expectation, and you will have seen the papers released already under the humble address, that coming from number 10, that he needed to be in post and in America as quickly as humanly possible.
"The very first formal communication of this to my predecessor from number ten, private office being that they wanted all this done at pace and Mandelson in post before inauguration. So that's the situation I faced."