Tory rebels and opposition MPs have cleared the first hurdle in their attempt to pass a law designed to prevent a no-deal Brexit.
Commons Speaker John Bercow granted them a debate and a vote on taking control of the Commons agenda.
If successful, they would be able to bring forward a bill seeking to delay the UK’s exit date beyond 31 October.
No 10 officials have warned the prime minister will push for an election if they succeed.
The debate – granted under a Commons rule to allow urgent discussion – can last up to three hours, with the main vote expected about 22.00 BST.
If the MPs win the vote – defeating the government – they will be able to take control of Commons business on Wednesday.
That will give them the chance to introduce a cross-party bill which would force the prime minister to ask for Brexit to be delayed until 31 January, unless MPs approve a new deal, or vote in favour of a no-deal exit, by 19 October.
Earlier, Conservative MP Phillip Lee defected to the Liberal Democrats ahead of the Commons showdown.
His defection means Boris Johnson no longer has a working majority.
In a letter to the prime minister, Dr Lee said Brexit divisions had “sadly transformed this once great party into something more akin to a narrow faction in which one’s Conservatism is measured by how recklessly one wants to leave the European Union”.
More than 10 Conservative MPs rose to their feet in support of the emergency debate application, moved by colleague Sir Oliver Letwin.
MPs are asked to stand to show their support if there are audible objections in the Commons chamber.
Opening the debate, Sir Oliver said this week would be the last for Parliament to block a no-deal exit, before it was due to be suspended next week.
Labour leader Jeremy Corbyn urged MPs to support the move, also arguing it would be the “last opportunity” for Parliament to block a no deal.
“If we don’t take action today, we may not get another chance,” he said, adding the government had set the country on a “destructive” course.
But Commons leader Jacob Rees-Mogg criticised the Speaker’s decision to allow the vote on the Commons agenda, arguing it was “constitutionally irregular”.
Last-ditch efforts to get the Tory rebels on side have been taking place, but there are thought to be about 15 who have not been won over.
The government had hoped the threat of an election – and of deselection and expulsion from the party – would be enough to bring them into line.
The prime minister says he wants a negotiated exit from the EU, but insists the UK must leave in all circumstances by the latest deadline of 31 October.
Earlier, Mr Johnson told the Commons he would travel to Dublin on Monday for discussions with Irish Prime Minister Leo Varadkar, focused on proposed alternative arrangements to the Irish border backstop – a key sticking point in the negotiations.
But he said that if MPs succeeded in their plan to block no deal, it would force him to go to Brussels to “beg for another pointless delay” to Brexit and he would “never” do that.
“It is Jeremy Corbyn’s surrender bill. It means running up the white flag,” he added.
In a no-deal scenario, the UK would immediately leave the EU with no agreement about the “divorce” process.
Opponents believe it would harm the economy, cause severe disruption to travel and supplies of goods like food and medicine, and lead to a hard border on the island of Ireland.
Proponents insist, though, that any disruption would be short-lived and could be managed with careful preparation.
It seems right now – although there is still some arm twisting going on behind the scenes – that the government is set to lose the vote.
We are finding ourselves in the middle of a full-throttle confrontation between a Parliament that does not want to allow the country to leave the EU without a deal and a prime minister who secured his place in power promising he would always keep that as an option.
Both of them cannot be the victors here.
And they are both determined to win.