MPs were due to vote on Prime Minister Boris Johnson’s revised Brexit deal. But they have voted for an amendment which withheld approval until legislation implementing it had been passed.
That means that as a deal has now not yet been approved by MPs, Mr Johnson is obliged to seek a delay to Brexit until 31 January.
The prime minister has said he will not “negotiate” a delay with the EU but it is unclear what will happen next.
A law passed in September – the European Union (Withdrawal) (No 2) Act 2019, also known as the Benn Act – compels Mr Johnson to write a letter asking for an extension if a deal has not been agreed in the House of Commons by the end of 19 October.
Delaying the UK’s exit date requires an extension to Article 50, the part of the Lisbon Treaty which sets out what happens when a country decides that it wants to leave the EU.
It allows an initial two-year period for negotiations on a divorce – finalising a withdrawal agreement and drawing up the broad outlines of a future relationship.
Article 50 was triggered by then Prime Minister Theresa May on 29 March 2017, giving an exit date of 29 March 2019.
This date was extended twice, first to 12 April and then until 31 October, after Mrs May’s deal was rejected in successive votes in the House of Commons.
So how does the extension process work?
The UK makes a request
The UK cannot make a decision about extending Article 50 on its own – it needs the agreement of all 27 other EU countries (EU27).
That’s what happened when the original 29 March deadline was extended.
The EU’s decision
Assuming that the prime minister does at least send a letter asking for an extension, the EU is not obliged to say yes to the UK – although it has repeatedly stressed its desire to avoid a no-deal Brexit.
If the EU were to reject the extension request, that would give MPs a stark choice between Mr Johnson’s deal and no deal.
Revoking Article 50
It’s worth pointing out that Article 50 can also be withdrawn or revoked.
The UK can in theory do that without consulting anyone else. That would mean that Brexit would not happen and the UK would remain in the EU on the same terms it has now.
The Liberal Democrats are the only party to say that would they would revoke Article 50 without a referendum if they won a majority in a general election.
The European Court of Justice (ECJ) has ruled that a revocation should be “unequivocal and unconditional”, suggesting that the ECJ would take a dim view of any attempt to withdraw an Article 50 notification and then resubmit it again a short time later.