The Conservative’s majority in Parliament has been cut to zero after Phillip Lee announced he is joining the Lib Dems.
Boris Johnson had been working with a majority of just 1 after losing the Brecon and Radnorshire by-election.
Lee, the MP for Bracknell, had threatened to quit the party before the summer recess.
Speaking on a podcast with Tory MP Sam Gyimah, he had said: “At the moment I’m increasingly feeling politically homeless.
“The party I joined was the party of John Major and John Major, I think, is probably feeling like this judging by his contributions in recent weeks.
“I’m really not comfortable about my party pushing for no-deal Brexit without proper consent of the public.”
Pursuing a damaging Brexit
In an announcement after joining the Liberal Democrats, 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 Bracknall 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.
Manipulation, bullying and lies
“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.
“That is why today I am joining Jo Swinson and the Liberal Democrats.
“I believe the Liberal Democrats are best placed to build the unifying and inspiring political force needed to heal our divisions, unleash our talents, equip us to take the opportunities and overcome the challenges that we face as a society – and leave our country and our world in a better place for the next generations”.