Britain’s 27 erstwhile European partners will meet next week to discuss the UK’s future with the country itself locked outside the room. The constitutional ground-rules of our democracy are in contention as never before, with arguments raging about whether or not Westminster can block Nicola Sturgeon’s mooted Scottish referendum, or Ms Sturgeon can deploy procedural weaponry at Holyrood to frustrate the UK-wide referendum decision. Meanwhile the grave economic consequences are coming into view, with the Institute of Directors suggesting that a quarter of companies will cease hiring and a fifth may shift operations overseas.

What is so damaging for the orderly conduct of business is not only the prospect of a messy divorce from the continent, but an immediate political crisis. After David Cameron’s post-dated resignation, nobody knows who is in charge, still less what happens next. The Brexit brigade stormed to victory without a manifesto, or even agreement on what “leaving the EU” involves. Some, including Michael Gove, would cut loose from the single market, and damn the vast economic cost; others now concede the UK will have to stay in the market, even though that would betray the campaign promise to “take control” of regulation and migration. Wider pre-election promises have turned to dust at great speed. At sunrise on Friday, while the final few votes were still being counted, Nigel Farage conceded that it had been “a mistake” for Vote Leave to pretend that there would be an extra £350m a week for the NHS; at sundown, leading leaver Daniel Hannan MEP killed the other central campaign promise by conceding free movement of labour might continue. Lesser gimmicks, such as Boris Johnson and Michael Gove’s promise to scrap VAT on energy bills, were always going to be hard to honour with an economy that is set to slow; they seem entirely irrelevant when we don’t know who will be presenting the next budget.

A country in peril, without a functioning government, needs an office-ready opposition – to ask the awkward questions, warn against wild swerves, and force somebody on the government side to explain what is going on. Britain, however, is saddled with a second party so beset by schisms, that – even as the union strains, the government crumbles, and the economy teeters – the thing making the headlines is Labour’s disintergation. Hilary Benn’s “sacking”, a dubious description seeing as the shadow foreign secretary constructed his own dismissal, was followed by the self-sacrifice of no fewer than 11 shadow cabinet ministers by late evening on 26 June, a run of career suicide bombs all detonated with the single aim of forcing Jeremy Corbyn out, just nine months after the leftwinger secured an almighty mandate from party members, taking more than thrice the votes of any of his three rivals, from the party’s centre and right.

This is a dismal pass that Labour was always likely to reach, because the overwhelming bulk of MPs have, from the off, been convinced that Mr Corbyn would drive them off a cliff. Many members are understandably furious with parliamentarians who never allowed him a chance. But there is no escaping that the day-to-day work of a party leader is fronting the efforts of a team of MPs. And when he was initially backed by fewer than one in 10 of that team, and not much interested in compromises to win more over, it was a job he was always going to struggle to do effectively. After the attempted coup, Mr Corbyn may attempt to fill his top team with other MPs. But there is now a real question about whether he can function in the job at all.

The PLP putsch looks shoddy for various reasons. For those MPs who have done nothing but plot since September, moving against him now is opportunism pure and simple. For others, it is an emotional spasm, made in rage against the painful loss of Britain’s place in Europe. But Mr Corbyn did not ask for this referendum, and it seems perverse to blame him for David Cameron’s loss. While Labour’s pro-Europeans were convinced they would win, many rather admired his qualified call for a remain vote, hoping that it might win do more to win over Eurosceptics than EU cheerleading. Even now, they should pause and ask whether a smooth-talking Europhile would have done more harm or good in Newport and Barnsley last week. One can argue that either way, but now – with the votes counted – anything that looks like pro-European regime change could appear disdainful of the public. Even more fundamentally, third-way social democracy has been in crisis ever since the crash. After the spectacular failure of the “mainstream” candidates last year, Labour’s centre-right should have earned its way back into contention with a fundamental rethink. Sadly, it hasn’t done that.

Set against this is the continuing failure of Jeremy Corbyn to look serious about becoming prime minister. There are fair-minded MPs who despair at the fact that so many constituents can never envisage their leader in No 10. It becomes harder to ignore such concerns after the Brexit vote, amid speculation that the next general election, which had not been scheduled until 2020, may now be brought forward to this autumn. This is the context in which deputy leader Tom Watson, who has his own mandate, is demanding talks about “the way forward”.

These are perilous times for progressive politics, perhaps the most perilous since the 1930s. It is incumbent on everybody to think before acting. Jeremy Corbyn needs to look into his soul and ask himself if he really wants to be PM, and get out of the way if not. His critics need to get on the phone and establish whether or not they can build the strength in the grassroots to topple him. Some say members are moving post-referendum; others dispute this. If the members remain loyal, then the mutineers could wound but not kill. The result would be to split the parliamentary party and the movement asunder, which would give the Tories a free run on a Brexit plaform.

Make the wrong call, and the results could be as ruinous as in 1931, when Ramsay MacDonald clung to office by giving up on the Labour movement, and a party of government was reduced to a rump of 52 seats. A half-complete coup would be worse than a clean defenestration or uniting behind Mr Corbyn. If the activists have moved, his time is up; if not, it is the MPs who must back off. Labour has sometimes failed dismally at moments of national crisis, as in 1931. But at others – think 1940 – it has proved equal to the hour. With an intolerant right wing on the rise, every progressive must be hoping that, whoever prevails in the left’s faction fighting, it will be finished soon.