The Brexit process reaches what could be a make-or-break stage this week as Boris Johnson and MPs engage in an extraordinary test of strength.
Tory rebels are hoping that by the end of the week they will have forced Mr Johnson to admit he must extend the October 31 deadline rather than try to leave without a deal.
But if they fail, the PM could emerge strengthened – and if they win there is mounting speculation he could opt to call an election rather than bow to their will.
MPs and peers will start massing at Westminster on the last day of the summer recess.
Remainers are putting the final touches to their plans for taking control of Commons business and passing legislation to block No Deal.
Jeremy Corbyn has held a shadow Cabinet meeting in Salford as he finalises his tactics.
The government is mobilising its forces to resist, in what promises to be an unprecedented clash between the executive and the House.
The Commons formally returns. Speaker John Bercow is likely to give his response from the chair to Boris Johnson’s decision to prorogue Parliament in mid-September until October 14. He has already branded the idea a ‘constitutional outrage’.
Remainers are expected to table a request for an emergency debate under Standing Order 24. Mr Bercow is likely to approve the request, and bend procedural rules to allow them to use the time to table a business motion.
If the rebels win a crunch vote, they would be able to take control of the Commons order paper and pave the way for a short piece of legislation ordering the PM to seek a Brexit extension.
They could also add extra sitting days, with speculation the House might sit on Friday and through the weekend. There has not been a Saturday sitting since 1982, discussing the Falklands War.
Away from Westminster, a court in Edinburgh is set to consider a legal challenge to Mr Johnson’s proroguation plan.
Consideration of the Bill could start, potentially wiping out plans for the spending review to be presented and Mr Johnson’s first PMQs.
The Bill will need support from Tory rebels successfully to clear its Commons stages, but there appear to be more than enough willing to act to avoid No Deal.
On Thursday, the High Court in London is due to consider another judicial review of Mr Johnson’s proroguation plan, which could offer more drama.
Former PM John Major, Remain campaigner Gina Miller and Labour deputy leader Tom Watson are among those involved in the case claiming that Mr Johnson is acting beyond his powers to silence Parliament.
The Houses are not currently due to sit, but there is a fair chance rebels will try to speed the legislative process by working through the weekend.
The Lords looks set to present the biggest challenge to rebel hopes, with Eurosceptic peers threatening a huge fillibustering effort to stop the Bill going through.
If the law has not passed by the time the House prorogues – which could happen as early as Monday – it will be wiped out, leaving Remainers with little or no time to try again when Parliament returns in mid-October.
But if they manage to get a measure on the statute book ordering Mr Johnson to seek and accept an extension from the EU, he could opt to call an election rather than obey. October 24 is regarded by many as a possible polling day.