MPs will be called to Parliament for a special Saturday sitting in a decisive day for the future of Brexit.
Parliament will meet on 19 October after a crunch EU summit – seen as the last chance for the UK and EU to agree a deal ahead of 31 October deadline.
If a deal is agreed, Boris Johnson will ask MPs to approve it – but if not, a range of options could be presented.
The BBC’s Laura Kuenssberg says these could include leaving without a deal and halting Brexit altogether.
MPs will have to agree on a business motion in the Commons for the sitting to take place.
Assuming they do, 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 2 September that year, due to the outbreak of World War Two.
The last time there was a Saturday sitting was 3 April 1982, due to the invasion of the Falkland Islands.
The prime minister has said he is determined the UK will leave the EU on 31 October, despite legislation, known as the Benn Act, which requires him to write to Brussels requesting a further delay if a deal is not signed off by Parliament by 19 October – 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.
Labour leader Jeremy Corbyn said his MPs would “do everything we can in Parliament, including legislating if necessary, to ensure [Mr. Johnson] makes that application”.
“The idea that the prime minister will defy the law yet again is something that needs to be borne in mind,” he added, appearing to reference the unlawful suspension of Parliament last month.
But former Conservative MP Dominic Grieve, who now sits as an independent after rebelling over Brexit, said he was a “bit mystified” at the need for a one-off Saturday sitting.
“I realise we are in the middle of a political crisis, but it is not a political crisis which makes me think we could not be sitting on the day before or on the following Monday,” he told BBC Radio 4’s World at One. “The government simply has not explained itself.”