Tory rebels are set to defeat Boris Johnson, writes Robert Peston

Brexit history and constitutional history may be made at 10 tonight.

Because the number of Tory rebels is holding firm at around 20.

And that means Sir Oliver Letwin’s motion under Standing Order 24, which would have the effect of handing control of business in the Commons to backbenchers tomorrow, could well pass by around five votes.

Which in turn means that the Bill to ask EU leaders to delay Brexit until January 31, would almost certainly become law by Sunday night.

And that is why Boris Johnson would tomorrow try to force a general election - because he has said he will never ask the EU to delay Brexit beyond October 31.

But if all of that happens, there will be bumps on the road. And indeed the wheels may yet come off the bus driven by those wanting to stop a no-deal Brexit.

And Johnson may struggle to get the general election on the playing field of his choice.

First, the Tory rebels - in partnership with Labour, the SNP, Plaid, the LibDems and the Green - will win if only the usual Labour Brexiters, such as Hoey and Mann, vote with Johnson.

If a larger number of Labour Leavers defy Corbyn’s whip, Johnson would triumph.

To be clear, I regard that prospect as unlikely. But it is not impossible.

David Gauke (L) and Philip Hammond are among the most senior of the Tory rebels. Credit: PA

Second, Johnson needs a two thirds majority of MPs to have his election, under the Fixed Term Parliament Act.

And probably a majority of Labour MPs - though probably not Corbyn - are wary of a general election.

They so mistrust Johnson that they fear he will find a ruse to set polling day AFTER there has been a no-deal Brexit on October 31, even if he promises polling day will be October 14 (which he will).

So he will only get his election if he can prove beyond any doubt that polling day is October 14.

Equally, even if he does that, many Labour MPs - with or without the leadership of Corbyn - will try to amend the motion brought under the Fixed Term Parliament Act, or find another legislative device, to make sure the bill delaying Brexit becomes law BEFORE the election is actually called.

Jeremy Corbyn may want a general election but it is likely many within his party will be apprehensive. Credit: PA

And if that looked as though it would succeed, Johnson would have to cancel the election - because if he were to lead his party into an immediate general election with Brexit delayed, the Tories would probably be smashed to pieces by the Brexit Party.

So there are no certainties about any of this. I think the Tory rebels will win tonight, and Johnson will tomorrow get his snap early election. But I may well be wrong.

And in case you are wondering what the Tory rebels hope and expect would come to pass if they win tonight and then get their Brexit-delay enacted, well they assume either an acceptable Brexit compromise would be negotiated with Brussels - which truthfully they don’t anticipate - or they think (more likely) it will lead to a referendum.

Which in turn means the general election would be next spring or next summer, respectively.

Consider yourself warned.