Boris Johnson has lost his working majority after a Tory MP defected to the Liberal Democrats.
In a moment of high-drama, Conservative MP Phillip Lee crossed the floor of the House of Commons and joined the Lib Dem benches just as Mr Johnson began giving a statement on last month's G7 summit.
It came ahead of an attempt tonight by rebel MPs to try and take control of the Parliamentary agenda and block a no-deal.
Mr Johnson has said he will call a snap election if he is defeated by the alliance of cross-party MPs.
Speaking to Sky News, Mr Lee said: "It’s all about the people, it’s about my constituents, it’s about my country.
"What I’ve done is move to a party which I think can secure a better future for them, that’s why I’ve done it.
"For a number of Tory colleagues it’s been a difficult 12 to 18 months and I suppose it started to accelerate with the change of administration.
"I mean ultimately I came into politics because I wanted to make a positive difference to leave a legacy for future generations and when you’ve got an administration that’s hell-bent in delivering a no-deal Brexit I’m sorry that’s just not in the interest of future generations.
"So I’ve moved to a party which has always been at the forefront of trying to stop this nonsense."
Posting a resignation letter addressed to the Prime Minister, the Bracknell MP, who was a justice minister from 2016-2018, wrote on Twitter: "After a great deal of thought, I have reached the conclusion that it is no longer possible to serve my constituents' and country's best interests as a Conservative Member of Parliament."
In the letter, he said: "The Brexit process has helped to transform this once great Party in to something more akin to a narrow faction, where an individual's 'conservatism' is measured by how recklessly one wishes to leave the European Union.
'Perhaps most disappointingly, it has increasingly become infected with the twin diseases of populism and English nationalism."
After a great deal of thought, I have reached the conclusion that it is no longer possible to serve my constituents’ and country’s best interests as a Conservative Member of Parliament. My letter to the Prime Minister: pic.twitter.com/0QreSbSdwR— Dr Phillip Lee MP (@DrPhillipLeeMP)September 3, 2019
He added in a statement: "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. 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.
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.— Jo Swinson (@joswinson)September 3, 2019
"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."
Moments after he crossed the floor, Lib Dem leader Jo Swinson tweeted: "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."
She added: "I am delighted to welcome Phillip to the Liberal Democrats at this crucial time.
"He brings almost 10 years of Parliamentary experience and decades of professional expertise. He shares our commitment to prevent a disastrous No Deal Brexit, and to stop Brexit altogether.
"The Liberal Democrats are growing. Phillip follows both Chuka Umunna MP and Sarah Wollaston MP in bravely crossing the floor to join us.
"Today, we also welcome Jane Dodds MP to Parliament, following her recent victory in Brecon and Radnorshire. And these representatives join over 30,000 new members, who have joined the Liberal Democrats since our best ever results in the European Elections in May."
She later said in the Commons: “The Prime Minister has lost his majority with the honourable member for Bracknell joining the Lib Dems.
"Doctors like him tell me they want to stop Brexit because it would plunge the NHS into a 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?”
Although he did not respond to him losing his working majority, Mr Johnson replied: “In the last six weeks we’ve been able to announce another £1.8bn going to 20 hospital upgrades around the country in addition to the £34bn extra that the Government is putting into the NHS.”
Earlier today, Justine Greening said she would not stand for the Tory party at the next election , taking a swipe at Mr Johnson and accusing the Tory party of "turning into the Brexit party."