Theresa May had made a last-ditch plea to MPs to back her withdrawal agreement tonight ahead of the "meaningful vote", urging them to help "deliver on the democratic decision of the British people" ahead of Brexit.
However, at about 7.40pm it was confirmed that the Prime Minister had suffered a humiliating defeat, as 432 MPs voted down the plan.
Rejection of Mrs May's Withdrawal Agreement now give her until January 21 to set out her Plan B, which is expected to involve going back to Brussels to seek further concessions.
There has also been speculation that it could trigger a bid to force a general election by Labour leader Jeremy Corbyn, who has said he will table a motion of no-confidence in the Government "soon" after it is defeated on its central policy platform.
Moments before the crunch vote, Mrs May told MPs: "Parliament gave the people a choice, we set the clock ticking on our departure and tonight we will determine whether we move forward with a Withdrawal Agreement that honours the vote and sets us on course for a better future.
"The responsibility of each and every one of us at this moment is profound, for this is a historic decision that will set the future of our country for generations."
But the Labour leader called on MPs to vote down the agreement, saying: "This deal is bad for our economy, a bad deal for our democracy, and a bad deal for this country."
Mr Corbyn could use a point of order in the immediate wake of tonight's vote to trigger a no-confidence debate as early as tomorrow (Wednesday).
The "meaningful vote" took place in the House of Commons at the end of eight days of debate and two years of negotiation following the EU referendum of June 2016.