Be absolutely clear. Keir Starmer is in very deep trouble indeed. Perhaps belatedly, he himself grasps this. His team and his ministers knew it already. His party and the public get it too. For this deeply unpopular Labour prime minister, the words approval rating are a contradiction in terms.

The eruption of speculation about Starmer’s future this week may have taken people by surprise. Where did that suddenly come from? The short answer is that No 10 briefed the latest twist – that Starmer expects to face and defeat a leadership challenge – on Tuesday evening. The longer one is that the Starmer leadership issue has been steadily gaining traction and credibility among MPs since the summer. This story is not a Westminster confection. Politically, it is very real. Dismiss it at your peril.

No 10 was responding to the issue that is the talk of the parliamentary Labour party. The talk is, very simply, a reflection of what the public now thinks. The public’s judgment about Starmer is brutal: it does not want him. The Labour party has to decide whether it thinks the public is right or wrong. That’s a dangerous game to play, especially for a party that is now sometimes running fourth in the polls.

The Starmer question facing Labour is both straightforward and complicated at the same time. The straightforward version is that he is very unpopular and helping to drag Labour down. Get rid of him and perhaps things will improve. The more complicated one is that there is little agreement over how, when, and to whose benefit Starmer should be ousted, or what the political consequences would be. This is a moment for Labour to be careful what it wishes for. But the problem and the argument are not going to go away.

Talk to MPs and officials and the anxieties are deep and unmissable. Many Labour MPs desire the end – a new leader – but draw back from the means. It’s serious, chaotic and incoherent, says one member of the government. Lots of discontent, but no plan, says a veteran. It’s worse than I thought, admits a cabinet minister. Once politicians start talking like this, though, the sands really are beginning to run out, and the process can speed up fast. The question is not whether Starmer is toast or not, a former insider says. He is just toast.

The briefings are simultaneously an admission by No 10 that there really is a problem and also an attempt to call the critics’ bluff. A challenge to Starmer’s leadership would be reckless and dangerous, the briefings said. It could destabilise markets, disturb international relationships and plunge Labour into the kind of party infighting that the public hates. These are not arguments that can be lightly dismissed.

But they have backfired. That this was aimed at Wes Streeting on a day when the health secretary was scheduled to undertake the morning media “round” and to make a speech on NHS reorganisation, is beyond doubt. Starmer’s aides have this week tried to flush out Streeting’s leadership ambitions in much the same way as they successfully flushed out those of Andy Burnham in September.

The downside was that the briefings made the leadership issue public and gave Streeting a national platform to talk about it. Streeting used it to restate his loyalty to Starmer and to attack the atmosphere in Downing Street as toxic. No 10 may have hoped that all of this would bring Streeting into line, but it has ended up showing that Starmer, who was forced to condemn hostile briefings in the Commons today, seems not to be in charge of his own administration. Streeting, meanwhile, underscored that he is a more adept media performer than the prime minister, as well as a credible contender for his job. When the prize is the prime ministership, however, others will also be tempted.

Where this all leads in the long term is therefore clear. Starmer will struggle to hold on. He is unlikely to lead Labour into the next general election. Mapping the precise path to that outcome is trickier – and predicting what happens afterwards is trickier still. The budget in two weeks’ time could force a challenge. But it may prove more popular with the party than the Labour doom-loop assumes. A landslide defeat in Scotland, Wales and the English local elections next May seems a likelier trigger.

Like John Major in 1995, Starmer could decide to force the issue by resigning and standing again. But that did nothing to strengthen Major’s authority, and it would do little for Starmer’s either. A more likely scenario, but still full of uncertainties because there is no precedent, would be that 20% of Labour MPs – of whom there are now 405 – would try to trigger a leadership contest. This would then be thrown open to Labour affiliates and members. Such a thing has never happened when Labour has been in government.

Traditionally, Labour has been exceptionally loyal to its leaders in ways the Conservatives cannot match. With the arguable exception of Tony Blair, who had done 10 years at the top, no Labour leader – and certainly no Labour prime minister – has been forced out against their will in modern times. Until now, the attempt to solve a party’s problems by changing the leader has been a Conservative speciality, as Theresa May, Boris Johnson and Liz Truss all found out. It is a sign of how much has changed in the 2020s that the loyalty brake no longer holds the Labour party together either, and that Starmer’s ousting is so seriously canvassed.

It is true that past Labour prime ministers have occasionally faced serious internal challenges that threatened their leadership. None, though, came so soon after an emphatic election win as that which Starmer now faces. Gordon Brown, a far more skilled political infighter, successfully faced down an abortive challenge from within his own cabinet in 2009 by making clear he would not go without leaving blood on the walls. Longer ago, in 1969, Harold Wilson told a rally: “May I say, for the benefit of those who have been carried away by the gossip of the last few days, that I know what’s going on. I’m going on, and the Labour government’s going on.” Starmer will wish it was so easy.

It is not just Starmer who is fighting for his political life right now. It may also be the Labour party itself. One poll this autumn suggested Labour could collapse to under 100 seats next time round, its worst ever. While Labour MPs may dream of a cathartic leadership change that could revive the party’s faltering fortunes, the real lesson may be that both of the old parties that have dominated British politics for the past century are now disintegrating irreversibly. Neither Starmer, nor Streeting, nor any other wannabe successor, may be able to do very much about that.