LONDON • Hopes for a second referendum on European Union membership are rising in Britain, amid heightened uncertainty over Brexit. However, big hurdles remain - from the timing to legal complexities on both sides of the English Channel.

Prime Minister Theresa May is struggling to convince British lawmakers to back her Brexit deal - formally signed off by EU leaders about a week ago - in a key vote in Parliament on Tuesday next week.

If, as widely expected, it is voted down, what happens next remains highly uncertain. But the backers of a so-called "People's Vote" argue it opens up an opportunity to ask Britons to think again.

"There is a growing momentum behind the campaign for a second referendum," said Mr Constantine Fraser, an analyst at research consultancy TS Lombard.

"It will become a serious option on the table if, or more likely when, Theresa May's deal is voted down," he said.

In the latest instance of second referendum activism, the pro-EU Best for Britain group last Saturday launched a new advertising campaign on vans, targeting the districts of key MPs such as Labour leader Jeremy Corbyn.

The support of the opposition party, which has delivered mixed messages on the issue - arguing for all options to be left on the table - is seen as crucial to force another poll.

Mr John McDonnell, Labour's finance spokesman, fuelled hopes the leadership was moving closer to the idea by saying last Tuesday that it was inevitable the party would support a second poll if it could not force a general election.

The hopes of second-referendum advocates were further strengthened by EU president Donald Tusk last Friday.

Speaking at the Group of 20 summit in Argentina, Mr Tusk said a rejection of the deal by the British Parliament would leave just two options - no deal or no Brexit at all.

There are significant structural barriers to a second vote, according to analysts. "You would need the government to actually table a proposal, have a vote in favour of it, which would require cross-party support," Dr Nick Wright, a fellow in EU politics at University College London, told Agence France-Presse (AFP).

Mrs May has repeatedly ruled out halting Brexit or holding another vote, and it would be hard without her support. "It's not impossible," said Mr Fraser. "If it becomes clear that there's political pressure for it in Parliament, the government may have no other option politically."

Even if MPs did eventually coalesce around another poll, legal and practical problems loom.

Britain has legislated to leave the European Union on March 29 next year, after triggering Article 50 - the treaty mechanism used to exit the bloc - two years earlier.

It is unclear if the Article 50 process could be paused or reversed unilaterally by the government. Europe's top court is expected to rule on the matter in days.

Britain could also try to agree to a delay with the EU. "Whether the EU agrees to extend Article 50 will depend on why the UK is asking," said Mr Fraser.

Some analysts think Brussels would be open to a delay for another referendum, but not for further negotiations.

But with European Parliament elections in the spring, the bloc might favour only a few additional weeks, which might not be enough time to stage another poll.

"If it ever gets to that, the Europeans will only extend Article 50 until the European elections," a European diplomatic source told AFP.

AGENCE FRANCE-PRESSE