- Video report by ITV News Political Correspondent Libby Wiener
MPs have rejected Theresa May's Brexit deal in a 'last chance' third vote held on the day which had been earmarked as the day the UK would leave the EU.
After a heated debate, it lost by a majority of 58 - with 344 voting against the motion, compared to 286 voting in its favour.
It means the default position now is for the UK to leave on April 12, with or without a deal.
Speaking after the results of the vote were announced, the Prime Minister said the implications of the vote were "grave".
She added: "I fear we are reaching the limits of the process in this House."
Giving his analysis of the day's events, ITV News Political Editor Robert Peston said: "This is the mother of all messes.
"On April 12 the default position is we leave without a deal and because of the complexities of avoiding that, that is a very high risk."
Speaking earlier in the day, Peston said: "The default now is that we leave the EU on April 12 in just a fortnight with no deal and the cost to our prosperity, many in the government believe, many economists believe, would be very significant.
"On Monday, MPs will have another go at coalescing around a different route through this mess and the one possible option that many think they could support is a customs union."
He added: "You might think that this is a sensible, deliverable way forward, but be under no illusion, this would be a real challenge for the prime minister because it would go against the terms of the Tories' last manifesto."
The Withdrawal Agreement had previously been voted down twice, but was allowed to go before the House of Commons a third time as it did not include the Political Declaration.
The Withdrawal Agreement argues the terms of actually leaving the EU, including terms on future trade and the Irish backstop - while the Political Declaration sets out plans for a future trade and security relationship with the EU.
In her closing remarks of the debate before MPs headed out to cast their votes, Mrs May said the vote on her divorce deal had been "the last opportunity to guarantee Brexit".
Following the vote's result, the noise coming from thousands of pro-Leave demonstrators, who were in Parliament Square for a planned protest, was one of confusion.
Some cheered when the PM suffered another Commons defeat but others were left fearing it meant a new EU referendum was now on the cards and were frustrated by a delay.
The March to Leave started in Sunderland two weeks ago - it was joined by Nigel Farage near Tate Britain, which is less than a mile from the destination of Parliament Square.
Mr Farage celebrated the defeat of Mrs May's "appalling" agreement, saying he would rather have an extension to Article 50 or leave with no deal immediately.
He said if there were a second referendum, "we will beat them by a bigger majority".
He added: "It's about who we are as people - it's about our children and grandchildren.
"We will get our country back. We will get our pride and our self respect back."
A separate Make Brexit Happen rally, backed by Tommy Robinson, was held around the same time and place as the Farage rally but appeared to run longer.
Mr Robinson, the English Defence League founder who now advises Ukip, acknowledged the confusion to cheering fans in Parliament Square.
"So Theresa May has lost her vote. Many people will be asking what does that even mean," said the activist, whose real name is Stephen Yaxley-Lennon.
"It means we were betrayed. Today is supposed to be our Independence Day."
As darkness fell, some of his supporters marched to Downing Street.
His supporters chanted "we want our country back" and "we want Brexit", before turning on the Channel 4 television crew.
The Met Police confirmed on Twitter five arrests had been made at the Brexit protests.
The tweet read: "As of 21:00hrs five arrests have been made at the demonstrations in central #London today: x2 for assault, x1 drunk & disorderly, x1 for assaulting a police officer & x1 male arrested after being identified as wanted for an offence in Herts. All are in custody."
In the wake Mrs May's defeat, President of the European Council Donald Tusk took to Twitter to announce he would call a meeting of the European Council on April 10 - just two days before the new Brexit deadline.
Reporting from Brussels, ITV News Europe Editor James Mates commented on the likelihood of an Article 50 extension, saying "it's not going to be automatic".
He said: "The French have always been reluctant to accept extending Article 50."
On the other hand, however, he added: "In Dublin they are going to be more affected by no deal than anybody, they are definitely looking for an extension if Britain wants one."
- ITV News Europe Editor James Mates explains the reaction in Brussels:
Meanwhile, the European Commission said a no-deal Brexit was "now a likely scenario" - though the markets appeared to disagree.
After Mrs May spoke, Labour leader Jeremy Corbyn then raised a point of order, in which he called for Mrs May to step aside and for a general election to be held.
"The House has been clear this deal now has to change. There has to be an alternative found - if the Prime Minister can't accept that then she must go," he said.
"Not at an indeterminate date in the future, but now, so that we can decide the future of this country through a general election."
The sentiment was echoed by Tory MP and former Brexit minister Steve Baker, who is deputy chairman of the European Research Group.
He said: "[The deal] has not passed. It will not pass. I regret to say it is time for Theresa May to follow through on her words and make way so that a new leader can deliver a Withdrawal Agreement which will be passed by Parliament.
"This has been a tragic waste of time and energy for the country. We can waste no more."
And SNP Westminster leader Ian Blackford said the time was right to revoke Article 50 and hold a general election.
Meanwhile, Green MP Caroline Lucas criticised Mrs May's promise to step down if her deal was voted through, calling it "grotesque" that the Prime Minister had been "willing to sell out the country's future for the price of some Tory MP careers".
Speaking outside Parliament following the vote, Jacob Rees Mogg - who leads the European Research Group - said the rejection of the deal would be "bad news for Brexit".
He had said he planned to vote against Mrs May's deal - which he labelled "very unsatisfactory" - but told journalists he had come to believe that the "perfect deal" was no longer available, which spurred him to change his mind and vote in favour.
As he walked, he passed a number of pro-Brexit protesters who shook his hand and voiced their support for his position.