British MPs have narrowly voted to prevent the country leaving the European Union without a deal under any circumstances, setting up a probable lengthy delay to the already protracted Brexit process.

The UK heads into unknown territory after MPs voted 312 to 306  against a no-deal Brexit on Thursday morning with little more than two weeks until the March 29 deadline to leave the EU.

The decision came after two-and-a-half years of negotiations and two failed attempts by Prime Minister Theresa May to pass her unpopular deal.

MP will now vote on Friday morning on whether to ask the EU to delay Brexit, probably by months.

Thursday’s vote went further than the government’s own planned motion, which noted that parliament did not want to leave without a deal on March 29, the leaving date set down in law, but stressed that the default legal position was to leave without a deal unless one was ratified by Parliament.

The approved motion has no legal force and ultimately may not prevent a no-deal exit after a possible delay, but it carries considerable political force, especially as it demonstrated a substantial rebellion by members of May’s own party.

If Britain does seek a Brexit delay, it will require the agreement of all of the EU’s 27 members.

The EU would prefer only a short extension, with the deadline of EU-wide parliamentary elections due on May 24-26, although it is unclear that this would be long enough to solve the impasse in London.

EU Brexit negotiator Michel Barnier said the bloc would need to know why Britain wanted to extend talks and that it was up to London to find a way out of the deadlock. The EU said there could be no more negotiations on the divorce terms.

The vote will give some relief to investors and British businesses, who feared a no-deal Brexit would spook financial markets, dislocate supply chains and damage the world’s fifth largest economy.

Diplomats and investors agree the UK now has four main options regarding Brexit: a delay, passing Ms May’s at the last minute, an accidental no-deal exit or another referendum.

Ahead of Wednesday’s vote, the British pound headed for its biggest daily rise in 2019 as investors bet that parliament would resoundingly vote against a no-deal Brexit.

After MPs crushed her deal for a second time on Wednesday morning, Ms May said it was still the best option for leaving in an orderly fashion.

“I want to leave the European Union with a good deal, I believe we have a good deal,” she told Parliament.

Northern Ireland’s Democratic Unionist Party, which props up the Ms May’s government, said her Brexit deal was “dead” and should not be put to parliament for a third time.

“We have not given up the goal of an orderly exit,” German Chancellor Angela Merkel said, “but yesterday’s events mean the options have become narrower.”

-with AAP