When Theresa May asks the EU's 27 government heads for a Brexit delay on Thursday, they will reply "what's it for, Mme Prime Minister?"

And the problem she's got - we've got - is she doesn't know, as the junior Brexit minister Kwasi Kwarteng made crystal clear when questioned in the Commons yesterday.

All she has is her deal, rejected comprehensively by MPs twice and ruled as ultra vires for a third vote by the Speaker yesterday.

There is no Plan B, there never has been.

So those same EU leaders are almost certain - according to well-placed sources - to postpone the decision on whether to grant a Brexit delay and on how long that extension would be.

They will "ask for more clarity from the UK" on the purpose of a delay.

"After all there is still a whole week to go", my source said sardonically.

The decision on a Brexit postponement would be made while the UK is clinging on to EU membership by the tips of exhausted fingers, with hours to go before the official Brexit day of 29 March.

The UK is set to leave the EU on March 29. Credit: PA

But in order to provide that clarity - that the postponement is for a defined purpose - the prime minister would have to show qualities she has hitherto kept hidden: the ability to recognise she has made a serious mistake, turn on a sixpence and, with gusto, adopt a different path.

In practical terms, she would have to urgently and systematically seek the will of MPs for what kind of Brexit, if any, they would support.

This would not just mean starting a process of holding so-called indicative votes in the Commons on what kind of way forward would command a majority of MPs, it would mean completing most of this cumbersome exercise by Thursday 28 March.

To put this mildly, this prospect of the prime minister needing to act with such haste is effing scary.

I see little reason to assume it will happen. But it must.

Absent such a manifestation that the UK appreciates the gravity of crisis, the Brexiter ultras will have their dreams fulfilled.

The point is that, whatever you think has happened in parliament over the last few days, as a point of EU and UK law, the UK is still leaving the EU with or without a deal at 11pm on 29 March.

"Maybe no deal will win," says my informed EU source. That's not a joke.