Conservative MP Dr Phillip Lee, a former justice minister, quit the party to join the Liberal Democrats and suddenly left it without a parliamentary majority.
In a moment of high-drama in the House of Commons, Lee crossed the floor while Prime Minister Boris Johnson was delivering a statement on the recent G7 summit.
Even with an agreement with the Democratic Unionist Party - which cost former prime minister Theresa May as much as one billion pounds ($1.2bn) in extra funding for Northern Ireland - Johnson's parliamentary majority was just one.
Lee's departure means the government has now lost its working majority in the legislature, and Johnson will likely find it impossible to continue governing without an election to shake up the parliamentary arithmetic.
'Manipulation, bullying and lies'
"Over 27 years ago I joined the Conservative and Unionist Party led by Sir John Major," said Lee - a former medical practitioner - in a statement.
"Since 2010 I have had the privilege of representing the Bracknell constituency. The party I joined in 1992 is not the party I am leaving today.
"This Conservative government is aggressively pursuing a damaging Brexit in unprincipled ways. It is putting lives and livelihoods at risk unnecessarily and it is wantonly endangering the integrity of the United Kingdom.
"More widely, it is undermining our country's economy, democracy and role in the world. It is using political manipulation, bullying and lies. And it is doing these things in a deliberate and considered way."
Liberal Democrats leader Jo Swinson welcomed Lee to the party, posting on Twitter: "Welcome @DrPhillipLeeMP - you have joined us at the most crucial time. I look forward to working with you to prevent a disastrous Brexit, and to fight for a fairer, more equal society."
Labour leader Jeremy Corbyn took the chance to poke Johnson with a pointed barb after attacking the prime minister for trying to run a "cabal" from Downing Street in order to take Britain out of the European Union without a deal despite the costs.
"This is a government with no mandate, no morals, and, as of today, no majority," Corbyn said in the Commons.