LONDON - Britain could be plunged into a constitutional crisis if Prime Minister Theresa May loses her crucial Brexit vote on Dec 11, a leading political academic told Xinhua on Sunday.

Lawmakers will start a marathon five-day debate on Tuesday on the withdrawal deal agreed between May and the European Council a week ago.

With up to 100 Conservatives MPs threatening to vote against the deal, it will plunge Britain's roller-coaster process of leaving the European Union (EU) into unchartered waters.

So far nobody knows what will happen if May loses the vote, with leading politicians and political commentators putting forward a range of post-vote scenarios.

They range from May seeking a modified deal from Britain, to her resigning as prime minister to let a new occupant of 10 Downing Street to take over the Brexit relay race, to a snap general election.

Professor Anthony Glees from the University of Buckingham told Xinhua in an exclusive interview: "This is a political crisis which can, on Dec 11, become a constitutional crisis if May loses, as all the experts believe she will do. If it is not resolved properly and quickly it will explode into a crisis affecting everyone in the UK, old, young, rich, poor, employer and employee, the sick and the healthy."

The leader of the main opposition Labour Party, Sir Keir Starmer, said in a Sunday morning television interview that his party will call for a confidence vote in May's government if she loses the vote, with a snap general election a possible outcome.

To add to May's political woes, opposition parties at Westminster are on the warpath over a government refusal to publish advice given about the Brexit withdrawal deal by the country's top legal officer, Attorney General Geoffrey Cox.

Cox, who wrote the advice, will make a statement on Monday in the House of Commons on the advice he has given, with only a summary of the legal advice published.

The Democratic Unionist Party, whose 10 MPs give May's minority government its slender majority, has accused the government of having something to hide by holding back on the full advice politicians have demanded.

Behind the row over the advice is a suspicion by some politicians that it explains the arrangements in the withdrawal deal over the fate of the post-Brexit border between Northern Ireland and the Irish Republic.

A permanent solution to maintaining a frictionless border between the two parts of Ireland has yet to be agreed by Britain and the EU.

The fear among some politicians is that a temporary border arrangement, which May insists would only be for a short time, could be in place indefinitely as it would need approval from the European Union to remove.

The Sunday Times claims in a major report that Britain would be trapped indefinitely in a customs union with Brussels if MPs back May's Brexit deal.

"The government's top law officer ruled that the only way Britain could escape the backstop would be to sign a new trade deal, which could take years. But he warned Britain could remain trapped if those talks collapsed," said the newspaper.

Its report claimed the details have been confirmed to the newspaper by three serving cabinet ministers, including the former Brexit Secretary Dominic Raab, who resigned recently because of his opposition to May's deal with the EU.

On the last day of her visit to Argentina where she attended the G20 Summit, May said: "The next nine days are a really important time for our country leading up to the vote on this deal."

May has insisted she will press on with promoting her withdrawal agreement, saying it is the best deal for the country and respects the decision of the 2016 referendum when Britain voted to leave the EU.

She returned to 10 Downing Street Sunday to prepare for what will be one of the most important and intense periods in British politics in a generation.

Meanwhile Glees, who is director of the Center for Security and Intelligence Studies at the University of Buckingham, said it seems inevitable May will lose on Dec 11, with no more than 286 of the 650 MPs backing her, and the number of supporters decreasing on a daily basis.

May's withdrawal plan, said Glees, is a compromise which has split MPs in the House of Commons because it pleases neither side.

Glees acknowledged that despite what MPs say, nobody can be sure how they will vote on Dec 11.

On the question of what will happen, Glees told Xinhua: "We are moving now into totally uncharted waters. Mrs May will probably want to try again if she loses on Dec 11. Perhaps a stock market and currency slump would focus Conservative MPs' minds on the need to support her a second time. Perhaps she would dissolve Parliament and have a general election."

Glees says his understanding is that May believes the British people voted to quit the EU and if she does not achieve this it will destroy the Conservative Party.

In a reference to the long standing Eurosceptic wing within the Conservative party, Glees said: "The Brexiters have, after 40 years, finally won and May has to recognize that. At the same time, as a vicar's daughter, she genuinely wants not to harm ordinary British people."

Glees said if it ends in disaster for May, and she faces a confidence vote it could lead one of her ministers taking over as prime minister.

"Or there could be an early snap general election and a new Labour leader, perhaps with my former Brunel student John McDonnell (Labour's Shadow Chancellor) in Downing Street.

"Britain could then enter a long period of political and economic decline, ending in state socialism in our country or a period of rule by experts and security chiefs."

However, it is also possible, believed Glees, that May will decide there needs to be a second referendum.

"It is inconceivable that a government, or a political party that induced widescale economic collapse in the UK could survive," said Glees.

The latest YouGov poll, carried out for the People's Vote campaign, shows support for remaining in the EU at 55 percent, the highest level by the polling company since the 2016 referendum.