MPs will be recalled on 19 October, even if there is no agreement to vote on – and forced instead to choose between crashing out of the EU 12 days later or extending the Article 50 process.
A Saturday sitting had been expected if a deal was struck at the Brussels summit two days earlier, to rush it through the Commons, but will now be staged “either way”, The Independent has been told.
With a deal now virtually impossible, with talks between the UK and EU on the brink of collapse, Mr Johnson will instead attempt to shock MPs into sanctioning a no-deal Brexit.
The prime minister could also reveal how he plans to circumvent the Benn Act, which will force him to send a letter to Brussels requesting the extension.
Downing Street has previously suggested he would fight the Act in the courts, but a final, dramatic showdown in parliament now appears to be the favoured option.
Votes could also be staged on whether to stage a Final Say referendum as the best route out of the crisis, or even on revoking Article 50 to cancel Brexit altogether.
Mr Johnson is unlikely to win a vote for a no-deal – he has yet to win a Commons vote on anything – but defeat would at least strengthen his claim to have been forced into an extension by anti-Brexit MPs.
It would be the first Saturday sitting since Argentina invaded the Falklands Islands in 1982 – and only the third since the Second World War.
The debate will also take place to the backdrop of another huge demonstration demanding a fresh Brexit referendum, expected to attract hundreds of thousands of people.
The ‘Let Us Be Heard’ event will be the fourth demonstration organised jointly by The Independent’s Final Say campaign and the People’s Vote organisation. The last one, in March, attracted up to one million marchers.