'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 in the House of Commons.
He said the government was "pursuing a damaging Brexit in unprincipled ways... putting lives and livelihoods at risk".
Mr Johnson has vowed to leave the EU on 31 October with or without a deal, but a number of MPs against no deal have come together across party political lines to try to stop it happening.
They have submitted a motion for an emergency debate to Commons Speaker John Bercow, and if successful, they will bring forward a bill that 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, 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".'
Read more: Brexit: Tory MP defects ahead of crucial no-deal vote
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