As soon as Jeremy Corbyn was elected leader this was predicted but the revolt by Labour supporters in favour of the Leave argument gave that prospect an immediate inevitability.
One frontbench member, Hilary Benn, has been fired and several others have resigned — eight at the last count.
The party, whose supporters have by far the most to lose by leaving the EU, is in disarray.
A central plank of Britain’s body politic is, at the very moment when it needs to be stronger, more reassuring and alert than it has been in 75 years, is split by bitterness, distrust and impotence.
It is hard to see how the Labour Party can have any real influence in the coming events that will reshape Britain and the rest of Europe.
Its constituency, which voted for secession because they felt ignored and marginalised will certainly be ignored and marginalised now.
They will, effectively, be without any meaningful or empathetic representation.
This cannot provoke anything but the greatest concern as it will exacerbate the anger that drove the unattractive strand of English nationalism so influential in the referendum.
The carnage on the other side of the House of Commons is more devastating.
While Corbyn’s party opponents are swinging a claymore like an over-enthusiastic Braveheart, extra David Cameron, as a parting gift to disloyal colleagues, has quietly slipped a stiletto between the ribs of the mendacious, clowning Boris Johnson and his Leave colleague Michael Gove.
He has fatally wounded their ambitions and may have turned their victory into something that may, in time, be seen as a defeat.
That defeat may not seem as great as the one suffered at the Battle of Little Bighorn (or the Battle of the Greasy Grass as the victors know it) by George Custer, who was as ambitious and as loose with the truth as Johnson, 140 years ago this weekend but it may have more than a passing if slightly less bloody resemblance.
All through the campaign, Cameron warned that a Leave vote would “the next morning” trigger Article 50, the mechanism for progressing a divorce from Europe.
He has, on mature reflection, changed his mind on the urgency of that formality and decided that a poisoned chalice should be a bequest for those who will step over his political corpse on their way to Downing St.
This is hardball, down-and-dirty politics delivered in a masterful way.
As some of the implications of a Leave vote became apparent — falling markets, sterling losing ground, Scottish and Northern Ireland rebuttals, the unchanged and unchangeable need to observe all EU rules to gain access to the free market — the job of cutting the umbilical cord now seems far less attractive even for someone like Johnson who could easily match Hillary Clinton’s superhuman ambition.
And that’s not all. Scottish nationalist leader Nicola Sturgeon has suggested the Scottish parliament could block legislation allowing the UK to quit the EU.
The suggestion that the referendum result is not binding on the House of Commons, that it is just advisory, deepens the quagmire too.
Mr Cameron must consider the prospect of prime minister Johnson trying to have divorce papers endorsed by a divided parliament with some personal glee but political despair.
These events may seem as remote as a Downton Abbey plot to many Irish people but as Paschal Donohoe, the minister for Public Expenditure and Reform points out, Britexit will have a negative impact on our economy, so like it or not, we have skin in this game.
It therefore is utterly disheartening and worrying that the British political system, so assured of its own capabilites, is in utter disarray and dangerously dysfunctional at this time of great challenge.