Is this Labour's most serious crisis for 90 years?

The best that Labour can say about the by-election results is that they could have been worse.

But even so they confirm that Labour is going through its most serious crisis since it became established as one of the UK's two main parties in the late 1920s.

Opposition parties very rarely lose seats they already occupy in by-elections; the last time that happened was more than 30 years ago - until last night, when it happened in Copeland.

And the best that can be said about Labour's victory in Stoke is that it shows so-called "red" Ukip under Paul Nuttall is less of a threat to Labour than some feared.

But even so Labour's share of the vote fell in Stoke, from more than 39% to 37%, while UKIP and the Tories together polled 49.1%, up from 45.2%.

Again that is hardly a triumph, at this stage in the lifetime of a parliament - especially with a government facing a crisis in the health service, chronic underfunding of social care for the elderly, schools complaining of funding squeezes, and small businesses moaning about business rate rises.

Part of what has gone wrong is that many voters do not see Jeremy Corbyn as a credible prime minister.

But the problem for Labour MPs and members - and its the members who ultimately are sovereign in a leadership contest - is that they cannot agree on who, if anyone, would do better.

The horrible truth for Labour is that it's not just about Corbyn.

All over the developed world moderate left-of-centre parties are in meltdown, from France to the US - as the indigenous normally white working classes fear that they no longer represent them in the way they did.

The way that so many Labour voters outside London and Scotland voted for Brexit, against the official Labour policy, is one manifestation of that here.

And the bitter divisions between Labour MPs and members about how and whether the party should oppose the government's approach to Brexit is another sign of the breakdown in the party as the natural coalition between those on lowest incomes and metropolitan, supposedly intellectual middle-class voters.

Counting gets underway in the Copeland by-election. Credit: PA

The phrase "existential crisis" is chronically overused in politics these days.

But it is applicable to a Labour party struggling to make itself relevant and credible in the only issue that matters to many voters, Brexit.

The conditions are more propitious than they have been since Labour was created for a fundamental realignment of the structure of our party system - of Labour fragmenting in the way the French socialist party has fragmented, with its many soft-left, pro-European MPs, members and supporters taking their loyalty and votes elsewhere, probably to a new party.