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. BBC