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.
Talks are ongoing between the UK and EU’s negotiating teams ahead of the summit on 17 and 18 October, but the political tensions have been ramped up in recent days.
Mr Johnson submitted new proposals to the EU last week 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.
French President Emanuel Macron had said the EU would make its position clear on the plans by the end of this week.
But a row broke out on Tuesday after a No 10 source said a call between Mr Johnson and the German chancellor, Angela Merkel, had made a deal “essentially impossible”, claiming she made clear a deal based on his proposals was “overwhelmingly unlikely”.
Mrs Merkel’s office said it would not comment on “private” conversations.
But the President of the European Council Donald Tusk sent a public tweet to Mr Johnson, accusing him of playing a “stupid blame game” – a criticism echoed by a number of opposition parties in the UK.
On Tuesday night, Irish Taoiseach (Prime Minister) Leo Varadkar spoke to Mr Johnson on the phone for 45 minutes, and told broadcaster RTE it would be “very difficult” to reach an agreement before the end of the month.
However, the two leaders are expected to meet for further Brexit talks later this week.
This special sitting will be a huge day.
That is because it will be the moment when Boris Johnson either returns to chants of “hail the conquering hero” – if he manages to get this elusive Brexit deal – or, more likely, returns with no-deal and has to set out his next steps.
And we are hearing that No 10 may seek to seize the initiative by putting down a series of motions for MPs to vote on – in other words asking them do they want to leave with no deal, do they want to revoke Article 50 etc.
But at the same time that Boris Johnson wants to use that moment to try and grasp the initiative, it is clear the rebel alliance of opposition MPs also wants to seize the day.
They want to ensure that Boris Johnson sits down, gets out the Basildon Bond and writes that letter to the European Commission asking for a further delay.
So both sides are now poised to try and gain control of that Saturday to map out the next steps, assuming – and I think it is a fairly widespread assumption in Westminster now – that there is not going to be a deal.