Former Conservative MP Phillip Lee has quit the party to join the Liberal Democrats.
In a moment of high-drama in the Commons, Dr Lee crossed the floor while Prime Minister Boris Johnson was delivering a statement on the recent G7 summit.
It means Boris Johnson’s Government has now lost its working majority in the Commons.
As Mr Lee crossed the floor of the chamber the Prime Minister said: “I wish my honourable friend all the best”.
Liberal Democrat leader Jo Swinson has said Mr Lee will now sit as a Lib Dem MP.
In a statement Mr Lee said: “Over 27 years ago I joined the Conservative and Unionist Party led by Sir John Major.
“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.”
In a tweet Mrs Swinson said: “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.”
Fellow Lib Dem Chuka Umunna said he was “delighted” to welcome Mr Lee to the party’s benches.
“This defection which erases Boris Johnson’s majority underlines the @LibDems position as the biggest and strongest Remain party in the UK,” he tweeted.