Theresa May faces an appalling choice on Thursday night - press ahead with the vote on her Brexit plan on Tuesday, and risk a massive, crushing defeat, or postpone the vote and return to Brussels on Thursday, to beg EU leaders to amend the terms of the UK's divorce from the EU.

The leader of the DUP, Arlene Foster, has minutes ago poured buckets of cold water on Downing Street's preferred third way - namely encouraging MPs to amend the motion on her Brexit plan to enshrine limits in UK law on the duration or inevitability of the so-called Northern Ireland backstop.

Foster, who sees the backstop as driving an unacceptable wedge between Northern Ireland and Great Britain, said on Thursday that: "Domestic legislative tinkering won’t cut it.

"The legally binding Withdrawal Treaty would remain fundamentally flawed as evidenced by the Attorney General’s legal advice".

So Theresa May's whips might just as well pack in their attempt to find some kind of UK legislative fudge to temper the impact of the backstop.

When the DUP say they are having none of it, that means they are voting against her Brexit plan as it stands.

If by some miracle the whips do find a fudge to secure enough votes to see her deal pass, the DUP would then work day and night to bring down her government.

So either way she loses.

Which means, as I said, that she now has a simple choice: submit to defeat on Tuesday that - sources close to the whips tell me - could be a margin of loss of around 300; or beg the indulgence of MPs to postpone the vote in the hope that she can get better terms from the EU.

Now the most influential of Tory backbenchers, Sir Graham Brady - chairman of the 1922 committee - tells me there is a case for returning to Brussels.

He says that if the Attorney General is to be believed, a permanent backstop would be illegal under EU law, and numerous countries fear that the terms of the backstop give an unfair competitive economic advantage to the UK.

So it stands to reason, he thinks, that if the PM asked nicely, the EU would decide to put a time limit on the backstop - and that at least would satisfy many Tory critics of her Brexit plan, and perhaps the DUP too.

There is only one problem: EU sources tell me there is absolutely no chance of the EU reopening the Withdrawal Agreement; and the best the PM could hope for would be extra warm words from EU leaders saying they want the backstop to be temporary.

But warn words won't cut it for the DUP or many Brexiters.

And anyway there are plenty of Tory Brexiters and Remainers who hate the Political Declaration on the potential long-term relationship between the UK and EU as much if not more than they hate the Withdrawal Agreement.

So there is a risk of the PM going to Brussels, getting a piece of paper that seems to satisfy some of her critics and still losing the vote when it comes back to Parliament.

Which is why Downing Street is still telling me tonight that she does not want to postpone Tuesday's vote.

That said, I'll bet she does postpone it - though I fear for her, and her government, that such a postponement would be a deferral of the moment MPs tear up plan and instruct her to start again.