It looks as though the longest rumoured split in a major British political party since the creation of the SDP almost 40 years ago will happen this morning.

The reason I think this is because last light I texted the Labour MPs Chuka Umunna, Chris Leslie, Luciana Berger and Gavin Shuker asking them if they were holding a press conference this morning to announce the split, and none replied.

For what it is worth, I could also have texted Mike Gapes, Angela Smith or Ann Coffey among other critics of the leadership of Jeremy Corbyn.

The mystery is not why they are leaving Labour: they are all alienated from a Labour leadership that they see as being far too slow and lacklustre in cutting out the cancer of anti-Semitism, and they are furious that Corbyn has been (as they see it) ignoring the majority of Labour members and supporters who would like the party to back an EU referendum.

No. The mystery is why today, rather than in a fortnight or six weeks, when perhaps the UK’s EU destiny will be a bit clearer.

The point is that those running the People’s Vote campaign for a referendum have been desperately trying to persuade Ummuna and Leslie to delay their split - because they think if they were to leave the party now, that would entrench the reluctance of Corbyn and those close to him to back a referendum.

As and when Umunna and co. formally leave Labour, the call for a referendum will be closely associated with those who have set themselves up as the enemies of Corbyn and his socialist project. So the referendum-sceptics around Corbyn will tell him that conceding a People’s Vote would be to capitulate to those who want to destroy him.

So the big question for Umunna and the Labour refuseniks today is whether in leaving Labour because they want a referendum they are not in practice undermining the prospect of a referendum.

Robert Peston is ITV's Political Editor. This article originally appeared on his Facebook page