In the most significant vote, MPs backed an amendment that means they will be able to rewrite any motion the government brings to the Commons in the days after a defeat on Mrs May’s deal, giving Parliament the opportunity to prevent a no-deal Brexit.
History was also made as MPs voted to hold ministers in contempt of Parliament for refusing to publish the full legal advice on the Brexit deal. It is believed to be the first time an entire government has been censured in British constitutional history.
It is the worst evening in the Commons for any prime minister in 40 years, with the government suffering as many defeats in a single day as Gordon Brown did in his entire premiership of just under three years.
Mrs May was abandoned by former allies on her own side, as well as by the DUP MPs that give her government its parliamentary majority.
It means defeat in the crucial final vote in six days’ time is confirmed as an odds-on likelihood.
In the space of an hour, MPs inflicted three defeats, starting with a humiliating motion of contempt that forced the government to agree to publish legal advice it has spent months withholding.
Under the threat of possible suspension from Parliament, the Attorney General Geoffrey Cox looked on as Leader of the Commons Andrea Leadsom told MPs their demand was “irresponsible” and would “seriously compromise good government” if agreed to.
Ms Leadsom warned that breaking the convention against publishing government legal advice would harm national security and could even put troops in harm’s way.
But MPs ignored her appeal to “exercise caution”, first rejecting a government bid to have the row referred for a lengthy examination by the Commons Standard Committee, then voting by 311 to 293 to hold the government in contempt. Nine out of the ten DUP MPs voted in favour of the contempt motion.
Ms Leadsom immediately agreed to release the legal advice today, saying: “We’ve tested the opinion of the House twice on this very serious subject.
“We’ve listened carefully and in light of the expressed will of the House, we will publish the final and full advice provided by the Attorney General to Cabinet.”
Shadow Brexit secretary Sir Keir Starmer said the contempt vote was “a badge of shame” for the government.
He said: “It is of huge constitutional and political significance. Never before has the House of Commons found ministers in contempt of Parliament.”
He added: “By treating Parliament with contempt, the government has proved it has lost its majority and the respect of the House.”
The DUP’s Westminster leader, Nigel Dodds, said: “We signed up to the confidence and supply agreement to support the government on Brexit on the basis of shared priorities.
“We haven’t broken that. Theresa May unfortunately seems to be going down a road of delivering neither Brexit in its true form or keeping the United Kingdom together economically and politically.”
The Commons immediately turned to procedure for the debate on the Brexit deal, with the government ambushed by an amendment from the former attorney general, Dominic Grieve.
MPs voted by 321 to 299 in favour of his amendment allowing the Commons to change the text of any motion that follows a defeat on the proposed Brexit deal.
A total of 26 Conservatives voted against the government in the biggest rebellion of the current Parliament.
“Parliament has tonight asserted its sovereignty,” Mr Grieve said following the vote. “MPs are tonight starting the process of taking back control.
“No longer must the will of Parliament, reflecting the will of the people, be diminished.”
Liberal Democrat leader Sir Vince Cable said: “This is now a government on the ropes. Theresa May’s majority has evaporated and the credibility of her deal is evaporating with it.
“The Commons is now very likely to defeat the government again next week on the Brexit deal, at which point the country must be given a People’s Vote and asked to choose between the deal or remaining in the EU.”
Mrs May was left to begin pitching her deal to the Commons in the wake of the series of defeats, telling MPs to accept that compromise is necessary to “bring the country together”.
Admitting the Brexit debate had been “corrosive to our politics”, the Prime Minister insisted the “only solution that will endure” is one that addresses the concerns of both sides.
But Mrs May was immediately reminded of the opposition to the deal from large parts of her own party, with former foreign secretary Boris Johnson claiming the inclusion of the Irish border backstop meant the EU would have “no reason to take its foot off our neck” in future trade talks.
“This is a recipe for blackmail,” Mr Johnson said. “The Spanish will make a play for Gibraltar, the French will go for our fish and our bankers, the Germans may well want some concessions on the free movement of EU nationals, and so it goes on.”
Labour leader Jeremy Corbyn said the Prime Minister had achieved something “extraordinary” with her Brexit deal: “It has united both Conservative Remainers and Conservative Leavers and members of every opposition party in an extraordinary coalition against the deal.”
Mr Corbyn said the government had treated negotiations with Brussels “only as an exercise in the internal management of the Conservative Party”.
SNP Westminster leader Ian Blackford said Mrs May was “fighting for her political life”.
He appealed to MPs to recall the progress made in Europe over the past 80 years and the benefits of free movement of people, adding: “It’s not too late to turn back.”