Conservative MP Phillip Lee has defected to the Liberal Democrats ahead of a showdown between Boris Johnson and Tory rebels over Brexit.
Dr Lee, the MP for Bracknell, took his seat on the opposition benches as the PM addressed the Commons.
His defection means Boris Johnson no longer has a working majority.
MPs hoping to pass legislation to block no deal have cleared the first hurdle after Speaker John Bercow granted them an emergency debate.
That debate could last up to three hours, followed by a vote. 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.
Speaking in the Commons earlier, Mr Johnson told MPs he wanted a negotiated exit from the EU and insisted there was “real momentum” behind the talks with Brussels.
He said 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.
Asked to provide evidence of progress by several Tory MPs, he said he would not negotiate in public but reassured them he would give details of the UK’s proposals well before the end of September to meet a deadline set by German Chancellor Angela Merkel.
But he said the moves by MPs, including Conservatives, to pass legislation effectively blocking a no-deal exit on 31 October would “destroy any chance of negotiating a new deal”.
If the rebels succeeded in their aims, Mr Johnson said 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.
No 10 has said the prime minister will push for an election on 14 October if the MPs succeed in blocking no deal.
But asked if he might simply ignore them and press ahead with a no-deal Brexit regardless, he said: “We will of course uphold the constitution and obey the law.”
Last-ditch efforts to get the Tory rebels on side have been taking place, but BBC political editor Laura Kuenssberg said the first meeting on Tuesday morning between the prime minister and the group went “less than swimmingly” and was “less than cordial”.
Further discussions reportedly began shortly after the PM’s Commons statement.
There are thought to be about 15 confirmed rebels. 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.
Before Dr Lee’s defection, Mr Johnson only had a working majority of one in the Commons.
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”.
“Perhaps more disappointingly, it has become infected by the twin diseases of English nationalism and populism.”
He told BBC Radio 4’s PM the “bullying” of MPs opposed to no deal showed the “tone and culture” of the Conservative Party had fundamentally changed, and he knew of other like-minded colleagues who were also considering their futures.
Welcoming her latest recruit, Lib Dem leader Jo Swinson said they would work together to prevent a “disastrous Brexit” which would do untold damage to the NHS and other public services.
Dr Lee’s decision to cross the floor – following that of ex-Tory MP Sarah Wollaston last month – was greeted with cheers on the opposition benches.
Amid angry exchanges during the PM’s statement on last month’s G7 summit, Jeremy Corbyn urged the PM to “reflect on his choice of language” to describe the rebels’ bill.
The Labour leader said the UK was “not at war with Europe” and it was a no-deal exit which would see the UK “surrender” jobs, employment standards and social protections.
“His is a government with no mandate, no morals and, as of today, no majority,” he added.
The SNP’s leader in Parliament, Ian Blackford, said Dr Lee’s defection capped what he said was the “shortest-lived honeymoon period ever” for a new prime minister.
He said his party was ready for a general election at any time.
But veteran Tory Ken Clarke, one of those set to rebel later, said the PM’s strategy was to “set conditions which make no deal inevitable, to make sure as much blame as possible is attached to the EU, and as quickly as he can fight a flag-waving election before the consequences of a no deal become too obvious to the public”. BBC News