Prime Minister Theresa May is struggling to convince British lawmakers to back her Brexit deal -- formally signed off by EU leaders last weekend -- in a key vote in parliament on December 11.
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 Constantine Fraser, an analyst 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.
"I wouldn't say it's a probability, but it's a likelihood that's growing fast."
Second referendum call 'inevitable'
In the latest instance of second referendum activism, the pro-EU Best for Britain group on Saturday launched a new advertising campaign on vans targeting the districts of "key MPs like 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.
John McDonnell, Labour's finance spokesman, fuelled hopes the leadership was moving closer to the idea by saying Tuesday 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 on Friday.
Speaking at the G20 summit in Buenos Aires, Tusk said a rejection of the deal by the British parliament would leave just two options -- "no deal or no Brexit at all".
'It's not impossible'
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," Nick Wright, a fellow in EU politics at University College London, told AFP.
May has repeatedly ruled out halting Brexit or holding another vote, and it would be hard without her support.
"It's not impossible," noted Fraser.
"If it becomes clear that there's political pressure for it in parliament, the government may have no other option politically."
A cross-party group of MPs on Thursday laid down an amendment to May's EU withdrawal legislation in a bid to stop a no-deal Brexit emerging as the default fallback option.
The proposed amendment would hand power to lawmakers if her plan is rejected in the House of Commons -- and could potentially provide a legislative pathway for a referendum.
Labour's Shadow Brexit secretary Keir Starmer said it had his "full support", tweeting it was a "great amendment".
'It would be complicated'
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, 2019, 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 a delay with the EU.
"Whether the EU agrees to extend Article 50 will depend on why the UK is asking," said Fraser.
Some analysts think Brussels would be open to a delay for another referendum, but not for further negotiations.
Timing 'biggest barrier'
But with European Parliament elections in the spring, the bloc might only favour 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.
The timing was "the biggest barrier" for a second referendum, said Fraser, noting that it could take four to five months to prepare and carry out.
"There is a potential for a clash: even with the extension of the Article 50 process, you'd be tangled up in the European elections in May."
But Wright argued that at that stage, with Britain potentially poised to vote to reverse Brexit, the EU would likely prove flexible.
"It would be complicated, particularly in terms of timing... but I don't think the EU would say no."
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