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".