Tory MP Phillip Lee has crossed the floor of the House of Commons to the Liberal Democrats in the middle of a speech by the Prime Minister Boris Johnson.
The defection of Dr Lee, 48, who is the MP for Bracknell in Berkshire, has abolished the government’s majority of one, despite the support of 10 DUP MPs.
Dr Lee said, in a statement explaining his defection: "This Conservative government is aggressively pursuing a damaging Brexit in unprincipled ways."
Liberal Democrat leader Jo Swinson said the government had lost his majority with Dr Lee's defection and said he had done so partly because, as a doctor, he feared the NHS was not safe in the hands of Mr Johnson because of a no-deal Brexit and the possibility of it being handed over to US private health companies.
Several Tory MPs, including former Chancellor Philip Hammond, are set to support a motion on a bill in a vote on Tuesday night, which would take control of the Brexit negotiations away from the government.
Mr Johnson referred to it as “Jeremy Corbyn’s surrender bill” and he said he would not support it.
The Prime Minister said: “Everyone in this government wants a deal…but this House has rejected a deal three times.”
“This week we are intensifying the pace of meetings. Our friends in Europe can see we want to make a deal,” he added.
The British people voted by 52 percent to 48 percent to leave the European Union in a 2016 referendum.
Mr Johnson's predecessor, Theresa May, sought to obtain a Brexit withdrawal agreement but her deal was rejected three times by Parliament and she eventually resigned in May.
Mr Johnson won the ensuing Tory leadership contest and when he moved into 10 Downing Street he insisted he would push through Brexit before 31 October.
On Tuesday Mr Johnson repeated that his government was determined to leave the EU on 31 October "come what may", even if that meant without a deal.
"We will be ready," Mr Johnson added.
He said: “We promised the people we would get Brexit done. Enough is enough. They want the referendum respected. We will negotiate a deal and though I am confident of getting a deal but we will leave on 31 October without one if necessary. There will be no further pointless delay.”
MPs frequently interrupted Mr Johnson in a rowdy House of Commons and the Speaker, John Bercow, had to call for “order” repeatedly.