An election 'is coming, and soon', writes Robert Peston

I am finding it hard to capture the scale of the parliamentary battle that will start on Tuesday - because what is at stake is huge, complicated and shifting.

One of its more important combatants described it to me as a "once-in-a-century crisis".

Another told me it would not only decide how and whether the UK leaves the EU, but also how and whether Scotland breaks away from the UK.

A third said that it would be "extraordinary" if within just a few days the hostilities between MPs did not lead to a general election being called.

And a fourth thought that what happens in parliament - and in the law courts on Thursday when a case is brought about the legality of Boris Johnson's decision to suspend parliament for five weeks from next week - may permanently alter the balance of power between PM and MPs, between executive and legislature.

Some MPs have joined together to back a plan to stop a forced no-deal. Credit: PA

So the week in Westminster will - in a proper sense - be extraordinary.

It starts on Monday when the prime minister, accompanied by Michael Gove and Amber Rudd, will attempt to dissuade 21 Tory rebels - led by Philip Hammond and David Gauke - from siding with the opposition to legislate against a no-deal Brexit.

The PM may deter a small number of his estranged colleagues. But the chances are that most will put their opposition to no-deal ahead of loyalty to party and a new PM.

Michael Gove and Amber Rudd will attempt to dissuade 21 Tory rebels on Monday. Credit: PA

Then on Tuesday, when parliament convenes after the summer break, there will be an emergency debate - that could go late into the night - culminating in a vote that would allow MPs to seize control of the order paper, or the business of the commons, on Wednesday and probably Thursday too.

For backbench MPs and opposition parties to decide what legislation is debated is highly unusual.

But - just as before the initial Brexit day of 29 March - the Commons Speaker John Bercow has told opposition leaders he will create the opportunities for them to do this.

On Wednesday therefore there will be a debate on a bill that would - as I said a few days ago - delay Brexit till May next year.

This is the maximum delay that European Union leaders are likely to allow, because unless the UK resolves by then whether it is staying or leaving the EU, those leaders will find it impossible to set their new six-year budget.

EU leaders simply cannot afford - in a literal sense - to allow the Brexituncertainty to persist beyond next spring.

Thousands gathered in cities across the UK on Saturday to protest against Boris Johnson's Brexit stance. Credit: PA

If the bill passes on Wednesday or Thursday - and I reckon the opposition and rebels have the numbers, just - it will then go to the Lords. And although Tory Brexiter lords may attempt to filibuster and talk the bill out of time in the upper chamber, few MPs or indeed members of the government think they will succeed.

That means the legislation would come back to the Commons for its final stages, possibly next Saturday and Sunday - though an emergency weekend sitting would require MPs to pass a facilitating motion.

And if the Brexit-delay bill completes all its parliamentary stages, it would be ready to send to the Queen, the sovereign, for it to be signed by her and turned into law.

The Queen would sign the Brexit-delay bill if it completes all its parliamentary stages. Credit: PA

But what then?

The PM's tactics both during the parliamentary debates and immediately afterwards are being war-gamed on Sunday at his country residence Chequers by him, the chief whip, Mark Spencer, and his closest aides, Dominic Cummings, Lee Cain and Nikki da Costa.

There have been sotto voce threats from those close to Johnson that he would refuse to pass the Brexit-delay bill to the Queen for signature, or that he would simply ignore the bill, or that he would ask EU leaders for the mandated delay but would then encourage a friendly EU leader to veto his own request or veto it himself.

Maybe.

However none of his ministerial colleagues - or at least none of those who have given an opinion to me - think he would make good on any of those threats, because to do so would be to behave (in their view) like the kind of tinpot dictator that British prime ministers are supposed to despise.

Truthfully, because Cummings in particular is wholly focused on delivering Brexit and not on the reputation of Conservative Party or government, I am not sure the PM will take MPs' order - so there may well be a ratcheting up of the constitutional crisis by several notches.

Sir John Major, Businesswoman Gina Miller and Labour's Tom Watson are launching a joint legal challenge. Credit: PA

Who knows? No one.

But if the conventional view is correct, that the law is the law, then Johnson's only resort could be - and would be - to seek a mandate for his approach to Brexit (no deal, unless and until the EU blinks) in a general election.

Which parliament could in theory block. But Jeremy Corbyn has again on Sunday - in the Observer - written that he craves an election, and the SNP would be clinically insane not to vote for one (given the mood in Scotland).

"If parliament legislates to delay Brexit, the prime minister will call an election, in just a few days" said one of his close colleagues. "And I think that is the most likely outcome".

I'll be back here soon enough writing about the most likely date of that election.

But my own wargaming - on back of postage stamp, far less sophisticated than Cummings', unless the tales of stochastic analysis in a Pentagon-style war-room is his practical joke - tells me that since Johnson's shtick is as putative defender of 17.4m Brexit voters against MPs contemptuous of "ordinary people", Johnson would commit political suicide if he tolerates being taken prisoner in Downing Street by MPs.

Or to put it another way, it is not unreasonable for Johnson to see a vote for the Brexit-delay bill as a vote of no confidence in him.

And following such a vote, his best hope - perhaps his only hope - of both the Brexit he wants and any ability to govern in any meaningful sense would be an imminent general election.