What is the Grieve amendment?

Dominic Grieve has been at the forefront of efforts to ensure that MPs have a say in what happens next if Theresa May’s Brexit deal is rejected by MPs in the meaningful vote on 11 December.

Amendments to the EU Withdrawal Act have already ensured that if MPs vote against it – as looks highly likely – the government must return to the House of Commons within 21 days and “make a statement setting out how Her Majesty’s government proposes to proceed”.

In the summer, after a personal appeal from May, Grieve blinked rather than inflict a defeat on the government over whether such a motion could be amended.

On Tuesday, as MPs prepared to open five days of debate on the Brexit deal, he pressed the issue again and defeated the government, with the backing of 25 fellow Tory rebels.

What difference does it make to Brexit?

Whatever motion the government brings back to parliament – which could in theory happen as late as New Year’s Eve, but will most likely occur before parliament rises for Christmas – will now have to be amendable.

Backbench MPs who prefer a different future relationship with the EU – or another referendum – hope that will allow them to put their own proposals to a vote in the Commons.

It was notable that backers of the amendment included Nick Boles, who is spearheading moves to try to force the issue of a closer, Norway-style deal, and claims to have discussed the idea with several cabinet ministers. They could move quickly to table their alternative.

Does it make a second referendum more likely?

At the margins, yes – it remains unclear whether there is a majority for another poll, but it ensures that MPs will have the opportunity to test the opinion of the Commons once the government has lost the meaningful vote. Backers of a people’s vote hope that will allow Labour to swing behind the idea.

Couldn’t the government just ignore MPs?

Formally, perhaps. Steve Baker, a former Department for Exiting the EU minister, was keen to point out on Tuesday that any such motions would not be “legally binding”; but politically, it would be very difficult for the government simply to carry on regardless.

Completing the process of leaving the EU requires a key piece of legislation – the withdrawal agreement implementation bill – to be passed, and in recent days MPs have demonstrated their willingness to tie the government up in procedural knots if it appears to be ignoring parliament.

What does it mean for Theresa May’s government?

Her authority, already severely dented, is hanging by a thread. Rebels on the Conservative side supporting the amendment included six former cabinet ministers, including Damian Green, Nicky Morgan and Michael Fallon.

With defeat on the meaningful vote looking all but certain, the Tories appear deeply divided about the way forward, and the amendment was the third vote the government lost in a single dramatic day.

With the Democratic Unionist party’s Nigel Dodds, whose MPs the prime minister relies on for a governing majority, saying later that he would welcome a general election, it is unclear whether the Conservatives can muster a majority for anything.