SIR KEIR STARMER is poised on the edge of the political precipice as he struggled to shelter behind failings by officials in the scandal over Peter Mandelson’s security vetting.

Labour MPs sat in sullen silence as the Prime Minister told the Commons that Foreign Office mandarins had kept him in the dark that UK Security Vetting recommended that vetting be denied to the New Labour grandee who he had already named as ambassador to Washington.

But left MP Diane Abbott cut to the heart of the matter, asking Sir Keir to loud Commons cheers: “Why didn’t you ask?”

Downing Street’s strategy is to place all blame on the Foreign Office, whose top official Olly Robbins was sacked by Sir Keir last week over the scandal.

The floundering premier’s agony is only set to continue tomorrow when Mr Robbins gives evidence on the row to the key Commons foreign affairs committee.

He is apparently consulting lawyers over his dismissal and his associates believes he has been “thrown under a bus” by the Prime Minister.

Committee chair Emily Thornberry also spelt out the truth today, telling Sir Keir that for “certain members of his team the priority was getting Peter Mandelson the job and security was very much a secondary consideration.”

That was a clear reference to former chief of staff Morgan McSweeney, the premier’s top aide who has also been forced from office by the scandal.

Many regard it as beyond belief that the Foreign Office did not advise Mr McSweeney, at least, that Mr Mandelson, already publicly named as ambassador, had failed the vetting.

Since losing his post Mr McSweeney, a protege of Mr Mandelson in Labour’s factional wars, has revealed that he also lost his mobile phone, doubtless containing vital messages, in a street robbery last October.

Sir Keir repeated that the decision to conceal the outcome of the vetting from him was “staggering” and “incredible,” as so much of this fiasco has been from beginning to end.

Ms Abbott, in her Commons contribution, pointed out that Mr Mandelson had twice had to resign from government over his dealings with millionaires and billionaires, and that his personal history was well-known.

Reform MP Lee Anderson was removed from the Commons for refusing to withdraw an accusation that Sir Keir has “been lying” about Mr Mandelson’s appointment. And Your Party’s Zarah Sultana was also suspended and removed for calling Sir Keir a liar.

“I have a duty to my constituents to tell the truth and the Prime Minister is a liar,” she said.

And John McDonnell said that Mr McSweeney had been key to Sir Keir’s election as Labour leader and that “when he became prime minister, the reward for Mr McSweeney was control of No 10 and, for Mr Mandelson, the highest diplomatic office.

“And the message, that unspoken message to civil servants, was what Mandelson wants, Mandelson gets,” he said.

Mr McDonnell demanded that Sir Keir clear this “toxic culture” out from Labour and launch an inquiry into the activities of the right-wing factional grouping Labour Together, once headed by Mr McSweeney, including into its harassment of journalists.”

Sir Keir did admit that he had been “wrong” to appoint Mr Mandelson to the Washington post and apologised once more to the victims of paedophile financier Jeffrey Epstein, with whom Mandelson retained a lengthy, intimate and ultimately career-destroying friendship, for his decision.

Mr Mandelson has since been arrested on suspicion of misconduct in public office over messages revealed by the US government apparently showing him sharing official documents, while a member of Gordon Brown’s Labour government, with Mr Epstein.

The disgraced ex-peer has denied any criminal wrongdoing or acting for financial gain.

The big question is whether Sir Keir can survive the apparently endless Mandelson saga in Downing Street.

Liberal Democrat leader Sir Ed Davey accused Sir Keir of being “in office but not in power” of a “catastrophic error of judgement” and urged him to resign.

Labour MPs however are holding fire until after the elections on May 7 for devolved authorities in Scotland and Wales and local councils across England, which are predicted to be anything from dire to disastrous for the Labour Party.

There appears to be limited appetite for a leadership election in the midst of the Iran war crisis and the two front-runners both have their difficulties.

Former deputy premier Angela Rayner has yet to resolve her tax affairs over the purchase of a flat in Hove, while Health Secretary Wes Streeting is tainted by his own close association with Mr Mandelson.

However, a contest sooner or later appears inevitable and delay could work to the advantage of Manchester Mayor Andy Burnham, who needs to return to the Commons before he can mount a challenge.

Many on the Labour left prefer Mr Burnham to Ms Rayner as successor, a position which argues in favour of not bringing the matter to a head in the short term.

Scottish Secretary Douglas Alexander could only muster the most tepid backing for the Prime Minister today, saying “I think he will” when asked if Sir Keir would lead Labour into the next general election.

“There are no certainties” he added, before explaining enigmatically that “as a Scottish Presbyterian I don’t believe in papal infallibility, nor do I believe in prime ministerial infallibility.”