Prime Minister Theresa May's Brexit deal has been rejected by 230 votes - the largest defeat for a sitting government in history.
MPs voted by 432 votes to 202 to reject the deal, which sets out the terms of Britain's exit from the EU on 29 March.
Labour leader Jeremy Corbyn has now tabled a vote of no confidence in the government, which could trigger a general election.
Mrs May said she would make time for a debate on the motion on Wednesday.
Mr Corbyn said the confidence vote would allow the Commons to "give its verdict on the sheer incompetence of this government".
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Playing the numbers game is dangerous - one seasoned politician who's been tracking every single move guessed at 90 against the deal this afternoon.
Other wilder claims of up to 200 against are doing the rounds too.
What you won't find today is anyone in Westminster who thinks the numbers are going to flip towards the government enough.
And that's why, although the speculation is enough to drive some people round the twist, much of the whispering today isn't about the vote itself, but about what happens next.
As I wrote last night, the meaningful vote will have multiple meanings.
But it seems clearer now what the prime minister's intentions will be in its immediate aftermath.
And guess what? Plan B is likely to be a tweaked version of Plan A.
For weeks cabinet ministers have been saying privately they expect Theresa May to try to hold broadly to her compromise.
It is not just the government's deal, but the EU's deal, a deal that has the backing of most of the big business groups, and indeed the product of more than two years of work.
In normal times a convincing Commons defeat would spell the end.
But it's become more and more obvious, and now it's on the record, that No 10 want to try to stick, to start with.
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Today, the leader of the Commons Andrea Leadsom firmly told us it's for the EU to budge towards the UK position (realistic or not), and the attorney general told MPs on the record in the Commons that the agreement will have to come back in much the same form - to count just two members of the government who are saying similar things.
It's very unlikely therefore that we are about to see immediately any dramatic pivot from the prime minister, even if the defeat is a resounding one.
MPs inside the chamber who are demanding that she switch course, and protesters on both sides who have turned Parliament into a ghoulish carnival (complete with a model of the Titanic, an Irish leprechaun and more union jacks and EU flags that you can count), are very unlikely to get their way tonight.
There is chatter around the cabinet that a tweaked plan would include a sunset clause on the controversial backstop could be approved by Parliament at some point next week, then be presented back to the EU as the only way through.
Mrs May is no stranger of course to sudden changes of heart.
These sentences may not age well. But I wouldn't bet much on today's undoubted drama provoking a major change of direction.
What's the point then holding this vote at all? You'd be forgiven for asking.
At some point Parliament's actual view had to be tested, and determined, not just discussed.
Some in government hope that a defeat today will allow MPs to get that need to express concern out of their system, then they will in time shift to back the deal.
But it may be a forlorn hope.
One MP likened today's vote as tearing off the plaster. It doesn't hurt any less when it is done slowly, and only once it's off can you know how much healing there is to do.