Jeremy Corbyn has said he would back a second EU referendum if Labour members demand he do so at his party conference.
The four-day gathering gets under way in Liverpool today in the wake of another Brexit setback for Theresa May, whose Chequers proposal was widely rejected by EU leaders in Salzburg earlier this week.
Mr Corbyn believes a general election would be the best way to resolve the political crisis enveloping the negotiations, but has admitted he would be prepared to push for another public vote on Brexit.
Mr Corbyn told the Sunday Mirror: "What comes out of conference I will adhere to. But I'm not calling for a second referendum. I hope we will agree that the best way of resolving this is a general election.
"But I was elected to empower the members of the party. So if conference makes a decision I will not walk away from it and I will act accordingly."
His comments came as a poll found 86% of party members think voters should have the final say on the outcome of the negotiations, and 90% would now vote to remain in the EU.
The striking results came from a YouGov survey carried out for The Observer, which found that just 8% of Labour members are opposed to another referendum on Brexit.
Even in the North and Midlands, where many Labour constituencies voted Leave in 2016, there was overwhelming support - 86% and 88% respectively - for a second vote.
Some 81% believe their standard of living would get worse after Brexit and 89% said it would be bad for jobs.
Labour deputy leader Tom Watson - who will appear live on Sophy Ridge on Sunday on Sky News from 9am - told the newspaper: "Jeremy and I were elected in 2015 to give the Labour Party back to its members.
"So if the people's party decide they want the people to have a final say on the deal, we have to respect the view of our members and we will go out and argue for it."
The prime minister has repeatedly insisted that there will not be a second referendum and has told warring Tories that the party must "hold our nerve" and unite in the national interest to deliver Brexit.
Whether or not Mrs May will strike a deal with the EU should become clearer at a summit in Brussels next month, but she has come under increased pressure from Brexiteer MPs to "chuck Chequers" and be prepared to walk away.
On Saturday, her former Brexit Secretary David Davis appeared at a lively Leave Means Leave rally alongside ex-UKIP leader Nigel Farage in Bolton.
Both criticised the Chequers plan put forward by Mrs May, but the prime minister has insisted it remains the only credible proposal on the table.
In a statement, Mrs May said: "Now is the time for cool heads. And it is a time to hold our nerve. I have said many times that these negotiations would be tough, and they were always bound to be toughest in the final straight.
"But what's also clear is that many in Labour, the Liberal Democrats and the SNP are trying to thwart Brexit at every step and seeking to exploit this moment for political gain.
"Some are now openly advocating a second referendum and extending article 50 to delay Brexit, sending us right back to square one. Others are talking directly to the EU to actively undermine the UK's negotiating position.
"But I say, this is the moment to put our country first. This is the moment to set aside our differences and come together in national unity. This is the moment to do what is right for Britain."
Her statement came amid reports that Downing Street was planning for a snap general election in an attempt to save the prime minister - something Number 10 has denied.
The Sunday Times claimed two senior aides were getting ready for a vote as early as November in a bid to win public support for the Chequers proposal, but a government spokesman said the report was "categorically untrue".
Mrs May will hold a cabinet meeting on Monday, which The Daily Telegraph has reported represents a "crunch point" in the Brexit talks, when ministers would "have to look again and reassess like Boris Johnson and David Davis did".
Work and Pensions Secretary Esther McVey and International Development Secretary Penny Mordaunt are among those who could resign if no backup plan is agreed to, the newspaper said on Saturday.
One cabinet secretary who has backed the prime minister is Jeremy Hunt, who has warned the EU not to "mistake British politeness for weakness".
Speaking to BBC Radio 4 on Saturday, the foreign secretary urged EU leaders to "step back from the abyss, to sit down and talk to us about how we can make these sensible, concrete proposals actually work".
He added: "If the EU's view is that just by saying no to every proposal made by the UK we will eventually capitulate and end up with either a Norway option or indeed staying in the EU - if that is their view then they've profoundly misjudged the British people."