MPs will be called to Parliament for a special Saturday sitting in a decisive day for the future of Brexit.
Parliament will meet on October 19 after a crunch EU summit, seen as the last chance for the UK and EU to agree a deal ahead of October 31 deadline.
If a deal is agreed, Boris Johnson will ask MPs to approve it, but if not, a range of options could be presented
These could include leaving without a deal, and halting Brexit altogether.
MPs will have to agree a business motion in the Commons for the sitting to take place.
If agreed, the additional day would coincide with an anti-Brexit march run by the People’s Vote campaign, which could see thousands of protesters heading to Westminster.
The House of Commons has only sat on four Saturdays since 1939, including on September 2 that year, due to the outbreak of World War Two.
The last time there was a Saturday sitting was April 3 1982, due to the invasion of the Falkland Islands.
October 31 deadline
The prime minister has said he is determined that the UK will leave the EU on October 31, despite legislation, known as the Benn Act, which requires him to write to the EU requesting a further delay if a deal is not signed off by Parliament by October 19 or unless MPs agree to a no-deal Brexit.
Scottish judges said on Wednesday they would not rule on a legal challenge from campaigners seeking to force the PM to send the letter or to allow an official to send it on his behalf if he refused. They said they would delay the decision until the political debate had “played out”.
No 10 has insisted Mr Johnson will comply with the law, but Laura Kuenssberg says there are still conversations going on in Downing Street about writing a second letter, making the case that a delay is unnecessary.
Talks are ongoing between the UK and EU after Mr Johnson submitted new proposals for a Brexit deal, centred on replacing the Irish backstop, the policy negotiated between Theresa May and the EU to prevent a hard border returning to the island of Ireland.
However, the EU has said there would have to be “fundamental changes” to the ideas put forward in order for them to be acceptable.
The UK’s chief negotiator, David Frost, will meet European Commission officials later, but sources on both sides said that technical talks had effectively reached the limit of what they could achieve.
Home Secretary Priti Patel said the government had been putting in “very intense” work in recent weeks to get a deal, so “nothing is over”.
But while getting an agreement was still their preference, they were “absolutely clear” that the UK would leave the EU on October 31 “come what may”, she added.
Brexit Secretary Steve Barclay and the EU’s chief negotiator, Michel Barnier will have a lunch meeting on Thursday to discuss the state of play.