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.
The prime minister has said he is determined the UK leaves the EU on 31 October, despite legislation passed by MPs last month, known as the Benn Act, which requires Mr Johnson to write to the EU 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.
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.
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.