In the chaos of the twisty government U-turns before Thursday’s Partygate inquiry debate, MPs could be forgiven for losing sight of the significance of the moment.

Stepping back from the internal politics, the heart of the matter is: Boris Johnson’s own MPs put so much pressure on party whips that the prime minister will now be formally investigated over whether he is in contempt of parliament for lying over lockdown parties. According to the ministerial code, this is a resigning matter.

There have been many predictions about what may ultimately finish Johnson, but none have proved right so far. Even after the prime minister was found to have broken his own laws so egregiously that he was fined – and he is expected to face repeated fines – still most MPs believe he should be given the benefit of the doubt.

But with an investigation by the privileges committee now certain, Johnson is once again in uncharted territory. The committee, which has six MPs who will examine the case after its Labour chair Chris Bryant recused himself, will be asked to form a view on whether what Johnson did was a resigning matter.

The temptation would be to dismiss this as yet another investigation into the same issue – but it may well have very different consequences. It opens up the possibility of MPs requesting the release of damaging new messages or photographs, though that is not guaranteed.

Its inquiry will produce a report to determine if Johnson was in contempt of parliament for having lied, and it can recommend punishments such as the suspension or expulsion of the prime minister.

All of that is unlikely in practice. The committee has a Conservative majority and is expected to be chaired by the senior Brexiter backbencher Sir Bernard Jenkin. Its final recommendation, if it included a censure, would require a Commons vote.

But it should also be said that many unlikely things have already happened – including Conservative whips being forced by their own MPs to allow this inquiry to take place in the first place.

Without a doubt, the inquiry will further prolong the agony of Tory MPs. But, most crucially, this third investigation into lockdown-breaking could be the one that leads to the narrow circumstance that Johnson has conceded could be a resigning matter.

Johnson has been bullish about remaining in post in the aftermath of the first two investigation into his behaviour – the Whitehall inquiry by Sue Gray and the Metropolitan police inquiry. Speaking to journalists en route to India this week, he said he would not resign in any circumstance and that he would fight the next election.

But this third inquiry is different. Johnson has previously acknowledged, in a roundabout way, that the ministerial code would compel him to resign if he misled the House of Commons.

At prime minister’s questions in January, the Labour leader, Keir Starmer, read out a clause from the ministerial code: “Ministers who knowingly mislead parliament will be expected to offer their resignation.”

Asked if that rule applies to him, Johnson said “of course” though he added: “Let me tell the House that I think he is inviting a question about an investigation … which he, as a lawyer, will know that I can’t comment on.”

That has proved to be a shrewd move from Labour – which has now paid off. Over the past 48 hours Labour’s tactics have also been sharp. Its motion anticipated that Tory MPs would require the new Partygate investigation to take place after the Met concludes its work, so it was included in its motion.

And Bryant struck a deal with Tory MPs, recusing himself from the privileges committee just as whips were using his chairmanship as a reason to oppose the inquiry.

There is wriggle room when it comes to misleading parliament, because the crucial word is “knowingly” and this forms a key part of Johnson’s defence. He told MPs “it did not occur to me” that the lockdown breach for which he was fined, a birthday gathering in the cabinet room in June 2020, was against the rules.

Yet the chaos this week suggests the prime minister’s party management is in worse straits than has been widely reported. With the new inquiry guaranteed to keep Partygate in the headlines for months, more and more MPs may begin to feel Johnson is running out of excuses.