Keir Starmer will deliver a high-stakes statement to MPs on Monday as he struggles to overcome fears inside his government that the Peter Mandelson vetting scandal could yet cost him his leadership.

In what is set to be a dramatic showdown, the prime minister will set out how Mandelson was able to take up his role as UK ambassador without the Foreign Office revealing it had overruled the decision to fail his vetting.

The scandal, first revealed by the Guardian last week, has already led to the sacking of the top civil servant at the Foreign Office, Olly Robbins, who is expected to appear before MPs on Tuesday in what could be another moment of grave peril for Starmer.

Ministers spent the weekend trying to shore up Starmer’s position after opposition party leaders called for him to quit over the affair, arguing he would not have gone ahead with sending Mandelson to Washington had he known.

But senior government figures are concerned that this week could be make-or-break for the prime minister – despite him being bolstered by his handling of the Iran crisis – if more damaging information should emerge or if sceptical Labour MPs should finally lose faith.

“We just don’t know how it will all play out, but all roads lead back to the original sin: Keir’s decision to appoint Peter Mandelson to Washington even though everybody knew it was high risk. This week could go either way,” one said.

Starmer said on Sunday he would make it “crystal clear” to MPs he had been in the dark over Mandelson’s vetting – and said it was “unforgivable” the Foreign Office failed to tell him after he had told MPs due process had been followed.

The prime minister will set out the facts on Mandelson’s security vetting after he instructed officials to urgently establish them when he was finally informed about the decision last Tuesday.

Downing Street also sought to demolish the argument from Robbins’s allies that he was prevented by law from telling ministers that Mandelson had failed vetting, arguing that there was a difference between being involved in the decision and being informed about it.

It published an explanatory document which said: “No law stops civil servants sensibly flagging UK security vetting recommendations, while rightly protecting detailed sensitive vetting information, to allow ministers to make judgments on appointments or on explaining matters to parliament.”

At the same time, Starmer told the Daily Mirror: “The fact that I wasn’t told that Peter Mandelson had failed his security vetting when he was appointed is astonishing.

“The fact that I wasn’t told when I said to parliament that due process had been followed is unforgivable, and that’s why I intend to set out in parliament on Monday the facts behind that, so there’s full transparency in relation to it.

“But am I furious that I wasn’t told? Yes, I am. Am I furious that other ministers weren’t told? Yes, I am. I should have been told, and I wasn’t told.”

Robbins, meanwhile, is understood to be taking legal advice after being sacked as head of the Foreign Office, and ahead of answering questions from the foreign affairs select committee on the scandal on Tuesday.

The former top civil servant is understood to feel angry at what he believes to be his unfair treatment by the prime minister, and determined to put his side of the story, after his dismissal sent a chill through Whitehall.

David Lammy, the deputy prime minister, told the Guardian that voters wanted stability, not “a self-flagellating, internally focused” governing party.

There has been frequent speculation that a leadership contest may ensue after what are expected to be brutal local and devolved elections for Labour on 7 May. However, in recent weeks, Labour MPs appeared to have stepped back.

“In the middle of 2026, with a serious global crisis affecting prices and affordability, the idea of jettisoning one leader because of a bad set of local results, it is just pie in the sky, if you think the electorate are going to thank us for that,” Lammy said.

“What will happen if we were to do that is that, very quickly, people would call for a general election. And I’m not sure that this is the moment to have one.”

Liz Kendall, the technology secretary, insisted Starmer had not gambled with national security by appointing Mandelson, as the government tried to limit fallout from the scandal.

Asked why he was not losing his job over the scandal, as a growing list of civil servants have done, she said it was because Starmer, “on the big calls facing this country, has made the right calls”, including on global issues.