(AFP) - MPs are set for a momentous third vote Friday on Prime Minister Theresa May's Brexit divorce deal, which could end a months-long crisis or risk Britain crashing out of the EU in two weeks.

The House of Commons has twice rejected May's withdrawal agreement, both times by large margins, but has been unable to agree any alternative -- and time is running out.

The pivotal vote takes place on the day Britain was supposed to leave the European Union until May asked the bloc's leaders last week for a little more time.

"We have the opportunity here to embrace certainty," Attorney General Geoffrey Cox told parliament at the start of the debate.

"We are at an important crossroads for the purposes of this nation's future and its history," he said.

In a last-ditch bid to garner the support of discontented Conservative colleagues, May dramatically promised Wednesday to resign if the deal passed.

She appealed to Brexit supporters to back her, saying that under the delay plan hatched with Brussels, approval on Friday could see Britain out of the bloc on May 22.

But if the deal falls again, she must set out a new plan to EU leaders -- with the options including a potentially catastrophic "no deal" Brexit as early as April 12, or a lengthy delay.

May's sacrifice swayed some of her critics, including former foreign minister Boris Johnson, a potential leadership contender.

Johnson had objected in particular to the deal's "backstop" provisions to keep the Irish border free-flowing after Brexit.

"It is very painful to vote for this deal. But I hope we can now work together to remedy its defects, avoid the backstop trap and strive to deliver the Brexit people voted for," Johnson wrote on Twitter.

But more than a dozen Conservative MPs still publicly oppose her deal.

May's Northern Irish allies, the Democratic Unionist Party (DUP), also continue to insist that its arrangements for the Irish border are unacceptable.

- 'Blindfold Brexit' -

Britain is leaving the EU after 46 years of membership following a divisive 2016 referendum in which voters decided 52 to 48 percent for Brexit.

But MPs appear incapable of agreeing on how to implement the result, reflecting the nationwide divisions that persist on the issue.

The ensuing chaos has led business leaders and trade unions to warn of a "national emergency".

May admits her agreement, reached last November during more than 18 months of negotiations, is a compromise but insists it is the best available.

It covers citizens' rights, Britain's financial settlement, plans for the Irish border and for a transition period to ease the split until new trade terms are agreed.

Without the support of her own side, May would have to rely on opposition Labour votes to get her deal through, but leader Jeremy Corbyn has vowed to vote against her.

The government has decided to put only one part of the Brexit package to MPs Friday, separating out the withdrawal terms from an accompanying political declaration on future ties.

Another vote on the political declaration, which is not legally binding, will be required for Brexit to happen.

Cox on Friday said that would happen "within the next few days".

He also told MPs that the EU was "open to negotiating a change" to the text of the political declaration, which sketches out an intention to retain close economic ties between Britain and the EU.

But Labour's Brexit spokesman Keir Starmer said separating the withdrawal agreement and the political declaration was "not part of a plan, it's a desperate measure," that the party would not vote for.

"Take the political declaration off and it's completely blind, you've no idea what you're really voting for."

The crunch vote caps a tumultuous week in Westminster which also saw MPs seize control of parliamentary business for a day to test support for various Brexit options.

However, none of the eight options drawn up by various lawmakers secured a majority, although they are hoping to narrow down the field in another round of votes next Monday.

Some of the views of MPs on the options, which included calls for much closer economic ties with the EU after Brexit, could be reflected in any new version of the political declaration.

MPs were not originally due to sit on Friday but the government called an emergency session, starting at 9.30am (0930 GMT) with voting due at 2.30pm (1430 GMT).

© Agence France-Presse