MPs have voted to block the UK from leaving the EU without an agreement.
In an unexpected move, an amendment tabled by Labour that toughened up the no-deal option, ruling out ever leaving the the union without an agreement, was passed by a four-vote margin.
This overruled the government's 'no-deal' motion that would only apply to the 29 March deadline and did not rule out the prospect of a no-deal exit later this year if Parliament is ultimately unable to agree a way forward.
This dramatic development led to the government ordering Conservative MPs to vote against its own motion.
But the government motion, now with the amendment attached, passed with an even wider margin of 43, reinforcing the message that MPs do not want to leave without a deal.
However the amendment is not a legally-binding decision - and it does not rule out the UK leaving the EU. The government could choose to accept it and treat it as binding but has not said it will.
At least one Conservative MP has resigned in the fallout from the vote, and there are government discussions as to whether other Tory MPs who abstained from casting a ballot - including Amber Rudd - will be forced to step down.
Under normal circumstances a Cabinet minister voting against the whips would result in them being sacked from their positions.
Mrs May had said before the ballot that she would vote for the government's no-deal motion, and that it would be a "free vote" - meaning party bosses would not tell MPs which way to vote.
Mrs May said the votes today showed there was a clear majority against no-deal deal.
But she said the legal default in UK and EU law was still that Britain would leave the EU on 29 March "unless something else is agreed".
"The onus is now on everyone of us in this house to find out what that is."
She said the options were the same as ever: Mrs May's deal; her deal subject to another referendum, which could lead to no Brexit at all; or seek to negotiate a new deal, but the EU has said the deal on the table is the only one on offer.
Labour Party leader Jeremy Corbyn said he would be working across the parties to find a compromise solution that could pass in the house.
MPs also today voted by 374 to 164 to reject a plan to delay the UK's departure from the EU until 22 May 2019, so that there can be what its supporters call a "managed no-deal" Brexit.
This amendment was proposed by Mrs May's former second-in-command, Conservative MP Damian Green, and was backed by prominent Conservative Brexiteers and Remainers.
It was known as the Malthouse Compromise - after Kit Malthouse, the government minister who devised it.
Yesterday, MPs in UK's House of Commons rejected British Prime Minister Theresa May's amended Brexit deal despite her managing to secure "legally binding" changes to it from the EU in 11th hour talks intended to soothe UK Brexiteers in parliament.
Tomorrow, MPs will get a vote on whether to request a delay to Brexit from the EU, and if all 27 member countries agree, the UK would not leave the bloc as planned on 29 March.
In 2016 the UK public voted in a referendum to leave the EU by nearly 52 percent to 48 percent - 17.4m votes to 16.1m - in 2016.