Boris Johnson

He has three options on that day. He can get a Brexit deal approved by the House of Commons, in which case we will leave the EU on or soon after 31 October (we may need a few extra days to get the necessary legislation through parliament).

If a withdrawal agreement is not approved, Johnson is required by law to ask the EU to extend the Brexit deadline. I have yet to see a plausible way round the law and I suspect Johnson and Dominic Cummings, his chief of staff, are bluffing when they suggest there is one.

Therefore, if there is no deal, Johnson has to ask for an extension or he has to resign in favour of an alternative prime minister who will. That means one of three apparently impossible things has to happen: a deal goes through the Commons; Johnson asks for an extension; or he resigns.

This week EU officials described the likelihood of a new deal as “pretty much nil”, which must be too negative, given Johnson’s desperation to secure a deal – any deal. But the numbers in the Commons still look daunting.

So if we get to 19 October without a deal, what will Johnson do? It seems unthinkable that he should give up the office of prime minister voluntarily. In the opinion of one former cabinet minister, he would ask for the extension and announce a referendum.

As ever, though, what would the question be? Labour has finally decided what the question would be in its referendum, but at the price of ridicule about Jeremy Corbyn negotiating a “sensible Leave option” against which he would then campaign. If Johnson put a deal that the Commons had rejected to a referendum, he couldn’t guarantee that parliament would vote for it even if the people did.

So I think he is more likely to resign as prime minister and declare that he wants to fight an election as leader of the Conservative Party. A temporary prime minister would have to ask for the extension, at which point the opposition parties would have no excuse for standing in the way of an election.

That doesn’t mean they would vote for one, though, especially if Johnson looked as if he might win. And there is some evidence that the failure to “get Brexit done” at the end of October wouldn’t damage Johnson that much, as long as the voters think he tried.

If it looks likely that Johnson would win an election, then, Labour MPs may continue to obstruct it, meaning he might be giving up the post of prime minister for a state of limbo – and Kenneth Clarke, if it is he, could be temporary prime minister for a while.

The only good outcome for Johnson is to get a deal through the Commons before – which probably means on – Saturday 19 October. Yet even if, like a comic book superhero, he escapes the closing walls at the last moment, he will only find himself in more trouble. If he “gets Brexit done” and we can finally talk about something else, it may turn out that what we talk about is the prospect of a criminal investigation into his time as mayor of London.

So far No 10 has dismissed the referral of allegations to the police as “politically motivated”. I don’t think it is, but even if it were, so what? Tony Blair was referred to the police – over the claim he had promised peerages in return for loans to Labour funds – by a Scottish National Party press release.

Nothing has gone right for Boris Johnson as prime minister. Even if the first stage of Brexit goes right for him – and that’s a remote possibility – there is plenty else that is ready to go wrong.