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 the Government has now lost its working majority in the Commons.
In a statement, Mr Lee said: “Over 27 years ago I joined the Conservative & Unionist Party led by Sir John Major.
“Since 2010 I have had the privilege of representing the Bracknell constituency.
After a great deal of thought, I have reached the conclusion that it is no longer possible to serve my constituentsâ and countryâs best interests as a Conservative Member of Parliament. My letter to the Prime Minister: pic.twitter.com/0QreSbSdwR— Dr Phillip Lee 🔶 (@DrPhillipLee) September 3, 2019
“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.”
Lib Dem leader Jo Swinson welcomed Mr Lee to the party.
In a tweet, she 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.”
Mr Lee was a justice minister from 2016-18.
Speaking in the Commons, Lib Dem leader Jo Swinson said: “The Prime Minister has lost his majority, with the honourable member for Bracknell joining the Liberal Democrats.
Welcome @DrPhillipLeeMP â 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.— Jo Swinson (@joswinson) September 3, 2019
“Doctors like him tell me they want to stop Brexit because it will plunge the NHS into deep crisis, haemorrhaging vital staff and threatening access to life-saving medicines.
“When will the Prime Minister stop playing with people’s lives and stop Brexit?”
Mr Johnson replied: “I’m glad you have given me occasion to remind the House there are in fact 700 more doctors in the NHS since the vote to leave the EU and, actually, just in the last six seeks we have been able to announce another £1.8 billion going to 20 new hospital upgrades around the country in addition to the £34 billion extra that the Conservatives are putting into the NHS.”