The EU has a choice to make on Thursday – assuming that the third “meaningful vote” (MV3) has not been tabled, debated and passed by then.  It could decide whether Theresa May is still Prime Minister this coming autumn.

The essence of John Bercow’s ruling yesterday is that the Prime Minister’s Brexit deal needs to be changed substantially before it is put to the Commons again.  His definition of substantial change suggests that texts on which the deal is based must themselves be altered if it is to be put to an MV3: that further legal glosses on them won’t be enough for him.

So if there is indeed no MV3 before the EU summit this week, the only practicable course open to May is to ask the EU for changes to the Withdrawal Agreement or, more likely, to the Political Declaration, or both.  As well as for the extension for which she will request in any event.

If the EU wants to carry on negotiating with a May-led Government, it will offer a short extension, ending before the European Parliamentary elections in June, and alter the Political Declaration, which essentially is not of a binding character, or even the Northern Ireland and Ireland protocol in the Withdrawal Agreement (i.e: the backstop), which is.

If, however, the EU has given up on the Prime Minister, it will offer no changes to the texts at all, or only further new legal documents – which will presumably not satisfy the Speaker that his conditions for holding MV3 have been met.  Plus a long extension.

Either way, May has not handled recent EU summits adroitly.  Last December, Jean-Claude Juncker dismissed her Brexit position as “nebulous”.  Last September, it gave her Chequers plan the thumbs-down.

The best bet is that neither the EU27 nor the Commission will decide their positions on the basis of which Prime Minister they want to deal with, but on preserving their position.  In particular, they will surely stick by Ireland.

In which case, the EU could conclude that a short extension would concentrate MPs’ minds wonderfully.  That would probably be wishful thinking.  Rather, the Commission and EU consensus position as we write seems to be that a long extension is more likely to get them a result. Perhaps the summit will finesse some muddled middle way – an extension until June, say, with the option of a further one.  That is sometimes the way of these events.

But whatever happens, May now faces a new challenge to her core aim: namely, preserving her premiership.  This is because an extension of any length would open the door to Oliver Letwin and company seizing control of the negotiation.  And open up time, too, for a Tory leadership election.  In such circumstances, the 1922 Committee and/or the Cabinet could finally force her out.

Downing Street may try to head off this threat to the Prime Minister’s position by getting the DUP’s support quickly, meeting the Speaker’s challenge, and putting MV3 to the test tomorrow evening. One suggestion is that May makes it clear that she will not allow Northern Ireland and Great Britain to diverge in regulatory or customs terms – a variant on the the so-called “Stormont lock”.

Whatever the merits or otherwise of such a proposal, it would be difficult for Bercow to argue that it did not represent a different deal to that put to the Commons for MV2 last week.  But betting on the Speaker being reasonable is not a gamble that this site would encourage anyone to take.

In any event, there is no sign that the Prime Minister would win MV3 this week even were she to get the DUP onside.  The “Spartans” are lined up and ready to oppose her deal at all costs.  There are almost certainly enough of these ERG and other Conservative holdouts to stop her deal getting through at any third attempt tomorrow.

If the Speaker’s ruling somehow leads to the EU offering concessions, and MV3 then passes next week, he may end up having done her a favour.  But it is more likely that it offers her nothing of substance – to use a Bercowian word.

George Eustice’s logic looks sound.  The EU will offer a longish extension.  The Commons will swallow it.  The Government will then table a statutory instrument to take the March 29 date out of the EU Withdrawal Act.  Both Houses will then rush it through next week – whether MV3 has been passed by then, or even put at all.

Yes, it is possible that No Deal could still somehow slip through some Parliamentary or timetabling or procedural gap.  But the odds against that happening are very long.