Theresa May warned MPs that “Brexit could be lost” unless they backed her deal, as she sought to persuade them to back her deal.

Meanwhile, she suffered a major political blow on Tuesday as Britain’s Attorney-General said that despite legally binding changes to the EU withdrawal deal, the legal risk of the U.K. being locked in an indefinite backstop with the EU remained “unchanged.”

The advice published by Geoffrey Cox QC on three changes to the withdrawal deal (not included in the withdrawal agreement itself) said the legal risk had been reduced but not eliminated entirely, and that the U.K. had no “internationally lawful means of exiting the protocol’s arrangements, save by agreement.”

“We could end up in a situation where we have no Brexit at all,” Ms. May warned MPs at a heated session of the House of Commons on Tuesday afternoon. While Mr. Cox called on MPs to back the deal, insisting it was a “political decision” for MPs to make, lawmakers across the political spectrum made plain their reservations with the deal.

Crucially, the influential European Research Group of MPs have said they can’t vote with the Prime Minister, as have the government’s supply and confidence partners, the Democratic Unionist Party of Northern Ireland.

A crucial vote is set to take place on Tuesday evening on the wider withdrawal deal in Parliament, in which the issue of the backstop has taken centre stage.

Should Ms. May lose the vote, she has pledged to allow a vote the next day on ruling out a no-deal Brexit. If MPs opt to rule out a no-deal Brexit, a third vote will take place on Thursday at which they will be able to push for a delay to Britain’s exit to the EU, due to take place in just over two weeks time.

Final round of talks

Amid predictions of a major defeat for Ms. May in the past couple of weeks, in a dramatic turn of events, she flew to the French city of Strasbourg on Monday evening to hold talks with Jean Claude Juncker, President of the European Commission, and in a joint press conference spelt out “legally binding’ changes.

These include a joint “interpretive instrument” stating that the EU could not act with the intent of applying the backstop indefinitely. If they did break the good faith, Britain would be able to challenge them through arbitration and could suspend the backstop. There’s also a joint statement with a legal commitment that the two sides would work to replace the backstop with alternative arrangements by the end of next year, and there’s a unilateral declaration from the U.K. that there could be nothing to stop the U.K. from instigating measures to get out of the backstop if the EU didn’t act in good faith.

“Having an insurance policy to guarantee that there will never be a hard border in Northern Ireland is absolutely right... But if we ever have to use that insurance policy, it cannot become a permanent arrangement and it is not the template for our future relationship,” said Ms. May.

However, speaking at a parliamentary select committee, Britain’s Brexit Secretary Stephen Barclay acknowledged that there had been no changes to the text of the agreement itself.

The developments add a new level of uncertainty to Tuesday night’s vote. While some are likely to be displeased with the lack of change, it still remains a possibility that enough MPs will vote with Ms. May, fearful of the votes that could follow on Tuesday and Wednesday that could rule out a no-deal Brexit, and lead to an extension.

However, Labour has said it will vote against the deal, as have the Liberal Democrats, the Greens, and the independent group of former Conservative and Labour MPs.

Additional steps

Should Ms. May win, there will still be a number of additional steps, including legislation that would have to pass through Parliament. With the deadline of March 29, even if Ms. May’s deal is passed, a technical extension might be needed, Mr. Barclay admitted.

What happens if she loses the vote is likely to be determined by the scale of the defeat. Should she lose narrowly, she could push for a brief extension to enable further discussions to attempt to get the deal through again. Should she lose by a large majority, it will be harder for her to push on with the current deal.

However, one thing is certain: change will have to come from the U.K. side. Ahead of the vote, Mr. Juncker urged MPs to vote for the deal.

Key issue

The Northern Irish backstop remains the key issue: the effective insurance policy would put the U.K. in a temporary customs union with the EU in the event of future talks breaking down in order to avoid a hard border developing on the island of Ireland and thereby maintain an open border there.

It would also require the U.K. to remain aligned with specific EU rules on a number of sectors. Complicating the situation have been the various objections of different political groupings in the U.K.: while the government is adamant it wants to exit the customs union, the right-wing European Research Group of MPs have all along wanted a guaranteed legal means for the U.K. to exit the backstop unilaterally to avoid becoming “trapped” in indefinite customs arrangement, which would prevent the U.K. from negotiating other trade deals.

However, the Democratic Unionist Party of won’t allow for any differences between the rules that Great Britain (England, Scotland and Wales), and Northern Ireland will be subjected to, which means the suggestion from the European Commission that they could allow Great Britain to exit the backstop unilaterally but not Northern Ireland has not been an option either.