Boris Johnson has repeatedly said the UK is leaving the EU on 31 October. But, by law, he is now effectively compelled to write a letter asking the EU for an extension.
Prime Minister Boris Johnson has repeatedly said the UK will leave the European Union (EU) on 31 October "deal or no deal".
So what exactly is this law and how might the government try to avoid another Brexit delay?
But under what circumstances could a no-deal Brexit still happen?
What does the Brexit extension law say?
EU leaders reject an extension
In September MPs passed a new law - introduced by Labour's Hilary Benn - designed to stop Mr Johnson pushing through a no-deal Brexit on 31 October.
Mr Johnson sent a letter to Brussels on 19 October, requesting a three-month Brexit delay. He was required to do so by a law - known as the Benn Act - after MPs had failed to approve his revised Brexit deal before that date.
Under the "Benn Act", Mr Johnson would be required to request a three-month Brexit delay unless he can pass a deal or get MPs to approve a no-deal exit by 19 October.
For this extension of the 31 October deadline to go ahead, leaders of the other 27 EU countries have to agree unanimously.
On 19 October, an amendment to Mr Johnson's new Brexit deal - put forward by expelled Conservative MP Sir Oliver Letwin - was passed saying MPs would not agree to a deal until legislation enshrining it in UK law was passed. That effectively brings the requirements of the Benn Act into force.
French President Emmanuel Macron, for example, has publicly questioned the extension - and without his support, it could not happen.
After the Letwin amendment passed, Mr Johnson said: "I will not negotiate a delay with the EU and neither does the law compel me to do so".
This would be the quickest route to a no-deal Brexit - because the deadline would stay at 31 October, leaving very little time for the government to implement its deal.
It's not yet clear exactly what he intends to do but what possible options might the government be looking at?
Extension agreed but UK fails to pass a deal
Two letters
If EU leaders agree to an extension, it does not take a no-deal Brexit off the table. Instead, it just pushes the possibility further into the future.
The law would force the prime minister to request the extension via a letter - it even sets out the exact wording.
Regardless of the length of the extension, the prime minister would probably still try to push his deal through Parliament.
One idea that has been floated is for Mr Johnson to follow the rules but write a second letter, telling EU countries that he actually does not want an extension.
However, if the government is unable to implement his deal (or another) before the new deadline, the UK would leave without a deal.
This could lead EU countries to decide not to grant the extension as it would be clear it is not what the UK government actually wants.
MPs reject alternative delay
In this scenario it is possible the EU could simply ignore the second letter and just grant the extension.
Although the letter from Mr Johnson asks for an extension of the deadline to 31 January 2020, EU leaders could propose a different date.
Persuade someone to reject
And according to the Benn Act, passed by Parliament in September, the prime minister would have to accept the proposed date - unless MPs voted against it within two days.
Any extension to the Brexit deadline would have to be agreed to by all EU member states. So, theoretically, Mr Johnson could comply with the law by writing the letter, but persuade just one country to veto it.
They would be unlikely to do so, however, for two reasons:
But persuading a country to side with the departing UK over their European neighbours could be a long shot.
Ministers refuse to change the deadline
MPs vote to reject an EU extension
This is the most controversial and unlikely scenario.
If the EU grants an extension but proposes a date other than 31 January 2020, MPs would have the power to reject the proposal, therefore leaving us heading towards a no-deal exit.
If an extension were agreed, a minister would be required to change the deadline in law using something called a statutory instrument (the power to change the law without a vote of MPs).
Ignore the law and wait for the courts
And, theoretically, they could simply refuse to do this.
The most simple option for the prime minister is to sit on his hands and refuse to write a letter. This would almost certainly lead to legal action and constitutional experts have suggested he would face almost certain defeat.
But not only would this be unlawful, it could be argued the "exit date" would be automatically changed anyway, because international law trumps domestic law.
Mr Johnson could bank on the legal process taking over 12 days to complete, which would take him past the Brexit deadline before being forced to ask for an extension,
Brexit explained
Refuse to resign
Brexit - British exit - refers to the UK leaving the EU. A public vote was held in June 2016 to decide whether the UK should leave or remain.
One option open to MPs who oppose a no-deal Brexit is to defeat the government in a vote of no confidence. In this scenario, an alternative government could emerge and take over from Mr Johnson if it can prove it has the confidence of a majority of MPs.
More news explainers
By convention, Mr Johnson would visit the Queen to resign and recommend the appointment of whoever the leader of the alternative government is.
In theory, he could refuse to leave and become a squatter behind the black door of No 10. This could put the Queen in an unprecedented position as she may be forced to sack him.
