Labour MPs frustrated with the lack of a clear mission from Keir Starmer’s No 10 have often urged the prime minister to be more forceful in his arguments, to prosecute his values, to find an enemy to define himself against.

The prime minister has found one: Olly Robbins. Starmer prosecuted his case against the former Foreign Office chief on Monday with the vigour of his former life at the bar.

He came armed with timelines and letters and the promise of a new inquiry. He insisted that, had he known Peter Mandelson had failed the vetting, his original sin of appointing him as US ambassador would not have been committed.

But what he did not deliver was any admission that he had inadvertently misled the House of Commons. That seemed too painful an admission for Starmer to make, when he had so expertly prosecuted Boris Johnson for misleading the House of Commons over partygate, when he had stood at the opposite dispatch box.

One senior MP, once loyal to the prime minister, described Starmer as having a chronic case of “good guy” syndrome, making it extremely hard for him to admit his failings. Starmer cannot fathom his own frame of mind when he agreed to Mandelson’s appointment.

So he blames the vetting system, his officials, his advisers and Mandelson’s own lack of candour. He is firm in his mind that he would never have made the appointment had he known about the failed vetting.

Perhaps that is correct. Allies of Starmer insist it is. But there was clearly a pervasive view in No 10 and the Foreign Office that this was an appointment where risks had to be taken; and Starmer has not yet publicly accepted that he was a part of creating that imperative which led to the overruling of so many warnings and conventions. “Appoint this man at all costs – but no, not like that!”

Most MPs arrived in Westminster on Monday with a sense of ennui rather than gunning for revolution. Even then, cack-handed pre-written questions handed out to MPs by whips almost accidentally lit the bonfire again.

In the emailed briefing, MPs were proffered useful quotes from the victims of Jeffrey Epstein which they could use in their questions in the Commons. One sample question quoted Rachel Benavidez on the importance of victims being heard, designed to turn the fire again on Mandelson’s conduct rather than the prime minister’s judgment.

The briefing enraged what was previously a sullen but resigned parliamentary Labour party (PLP). “I came in this morning prepared to hear him out and then they hand out this absolutely outrageous bollocks,” one MP said. “I don’t think this is what contrition looks like.” Another called it “despicable”.

But there was not the same raw anger that came on the day of the humble address motion that forced the Mandelson disclosures. An ire so intense it forced the resignation of Starmer’s chief of staff, Morgan McSweeney, and the Scottish Labour leader, Anas Sarwar, called for Starmer to resign.

Instead, those speaking from the government benches on Monday were mostly MPs who are serial critics: Rachael Maskell, Diane Abbott, Karl Turner, John McDonnell. Many other MPs had stayed away, campaigning for the local elections in their constituencies for an extra day. Few were prepared to read the whips’ friendly questions.

So, how at risk is Starmer now? Two things, MPs say, are simultaneously true. Firstly, it is not the right time to change the prime minister. Starmer was right on the call not to join the US offensive against Iran and it would be a risk to oust him as the country faces a potentially massive economic shock, particularly without any oven-ready challengers to command a majority of the PLP.

Secondly, they say, the fundamentals have not changed. Starmer is a deeply unpopular prime minister with a lacklustre vision of how to govern, and the party is flirting with fourth in the polls. This is not a sustainable position and eventually something will have to change. “I’d say it’s kicked [the odds of a] post-May challenge up from 4/10 back to a 7/10,” one party figure said.

“I think people buy the explanation of what happened. But it’s the broader picture that is the problem,” another said.

There is a major caveat to all of the above. On Tuesday at 9am, the witness for the defence will take the stand: Robbins at the foreign affairs select committee.

So far there is no hint that he will come before the chair Emily Thornberry with a smoking gun, but prime ministers have been undone by former permanent secretaries before. The former head of the Foreign Office Simon McDonald sealed the fate of Boris Johnson with proof that the then-prime minister misled MPs. There will be no saving Starmer if anything he said on Monday is found to have been on the wrong side of the line.