John Bercow, the Speaker, allowed a vote on a controversial amendment tabled by ex-attorney general Dominic Grieve, which aims to prevent the government from running down the clock to exit day on March 29.

The row stems from a complex bit of parliamentary procedure, which could have serious implications for the government. Here we explain why.

What is the Grieve amendment?

MPs are about to begin a five-day debate on the Brexit deal, ahead of a critical vote on Ms May’s plans on Tuesday. The vote was due to be held in December but the prime minister delayed the clash at the eleventh-hour in the face of certain defeat.

The debate is now being re-run, beginning on Wednesday and running through to Tuesday night, when the so-called meaningful vote will be held.

Under current rules, if Ms May’s deal is defeated in the Commons, the prime minister has 21 days, plus seven sitting days, to return to the chamber and offer MPs an alternative, or another vote.

But Mr Grieve, a leading Tory Brexit rebel, tabled an amendment to the business motion which would require Ms May to return within three days of losing the vote.

Cross-party MPs backed the move, defeating the government by 308 votes to 297.

Why are MPs so annoyed about it?

Conservatives are furious that Mr Bercow accepted the Grieve amendment, as parliamentary rules usually only allow a government minister to amend motions of this kind.

The Commons descended into chaos as Brexiteers accused the Speaker of being partisan and overruling long-held procedures. Labour MPs cheered and spoke in support of Mr Bercow.

It ties into long-running anger among some Tory MPs, who say Mr Bercow is too friendly towards Labour and pro-EU groups – as the Speaker is supposed to be impartial.

What does it mean for May’s deal/vote next week?

Mr Bercow made it clear that the motion does not carry legal force, but it does significantly increase the pressure on Ms May to speed up the Brexit process.

The prime minister is still expected to lose the vote next week, as she does not appear to have won fresh concessions from Brussels over the Christmas break that would convince her mutinous MPs to support the deal.

If she loses, the Grieve amendment means she has three working days to return to the Commons with new plans – limiting the amount of time she has to go back to the EU.

Does it increase the likelihood of a general election/second referendum?

The defeat is a fresh humiliation for Ms May and a clear sign of her weak grip on her party and the Commons. But she has endured defeats that would have toppled other prime ministers, so this is not enough to end her premiership.