Conservative Party MP Dr Phillip Lee, a former justice minister, has quit the party to join the Liberal Democrats, suddenly leaving Prime Minister Boris Johnson's government without a parliamentary majority.
In a moment of high-drama in the House of Commons, Lee crossed the floor while 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.
Al Jazeera's Neave Barker reports from London.