What does Dominic Grieve's debate amendment mean for Theresa May's Brexit deal?

The Dominic Grieve amendment intends to increase the power of MPs to decide what kind of Brexit, or no Brexit, will transpire. Credit: PA

The point of the Grieve amendment, that may be voted on Tuesday, is to make next week's vote on Theresa May's Brexit plan more "meaningful" - and to increase the power of MPs to decide what kind of Brexit, or no Brexit, will transpire.

The point is that right now if she loses the vote on her Brexit plan, as she is set to do, she has to bring back a Plan B to MPs within 21 days, but MPs have no power to amend or alter that Plan B.

But if the Grieve amendment is carried, MPs would at that point have the power to change her Plan B.

Now this I am afraid is where it all gets surreal and complicated. PLEASE BEAR WITH ME. For Remainers and soft Brexiters, Grieve's amendment eliminates the risk of a hard no-deal Brexit, because it would give MPs power to vote against a hard, no-deal Brexit.

And they believe probably correctly, that a majority of MPs oppose a hard no-deal Brexit. So the Brexiters of the European Research Group will vote against Grieve's amendment, because many of them want a no-deal Brexit, But they expect Grieve's amendment to carry given it is likely to be supported by Labour and other opposition parties.

So how would and should Brexiters vote next Tuesday if, as expected, Grieve's amendment carries? Should the Brexiters then vote with the government and make sure May's deal is carried - if no-deal Brexit has been taken off the table as an option?

My guess is that May and her whips think that is precisely what Brexiters would and should do - and that they secretly hope Grieve will win. However (and I told you this was mad and complicated) the Brexiters DISAGREE.

They say Grieve's amendment gives parliament the power to amend the government's plans but not in a BINDING way. They say the Remainers are wrong, that parliament could not actually take no-deal off the the table. All parliament could do would be to pass a NON-BINDING motion against no-deal.

Here is the moment of maximum pain for May. In practice Grieve's amendment could - COULD - make the majority against May's Brexit plan even bigger. Because it could increase the motivation of both Remainers and Brexiters to vote against her. I imagine that.

Some time this afternoon the penny will drop for May and her whips about all this, and at that point they will go hell-for-leather to try to kill Grieve's amendment. But make no mistake, if the Speaker allows a vote today on Grieve's motion, it will matter.