Jeremy Corbyn has suggested that a second referendum could follow the near-certain defeat of Theresa May's "worst-of-all-worlds" Brexit deal in a Commons showdown next week.
The Labour leader said "all options must be on the table" if MPs vote down the prime minister's blueprint on Tuesday, a defeat that could be so catastrophic that it threatens Ms May's leadership and her government.
Labour's preferred option is a general election but as the Brexit talks enter the chaotic final stages, senior figures such as shadow chancellor John McDonnell and shadow Brexit secretary Keir Starmer have paved the way for a shift in stance towards a new public vote.
The party has been riven with divisions over its approach to Brexit, as it tries to balance the views of pro-European supporters in cities and Leave voters in its traditional heartlands.
In an intervention just days before the meaningful vote, Mr Corbyn urged MPs to reject the prime minister's "botched Brexit deal".
Writing in The Guardian, Mr Corbyn said: "In the past, a defeat of such seriousness as May now faces would have meant an automatic election.
"But if under the current rules we cannot get an election, all options must be on the table.
"Those should include Labour’s alternative and, as our conference decided in September, the option of campaigning for a public vote to break the deadlock."
It comes as Mr McDonnell rejected warnings from influential union boss Len McCluskey that supporting a second referendum would be regarded as a betrayal by voters.
He argued that, if Labour's attempt to secure a general election fails and they are not able to negotiate a new deal, the public would understand the potential need for a second vote.
Speaking on a visit to Scotland, the shadow chancellor said: "I think people will recognise we would have no other option but to consider another public vote.
"People respect us for doing our best to implement the spirit of the referendum result."
Labour would "rapidly" reopen negotiations with Brussels, as the EU would be open to exploring other options, he said.
Mr McDonnell said: "We believe that next week, when Theresa May's deal is voted down, Parliament will have the opportunity to explore other routes.
"We've offered a route that could gain sufficient support and that does mean reopening negotiations rapidly.
"It happened with regards to the Lisbon Treaty when a couple of countries had a referendum and took a different view."
The Independent has launched its #FinalSay campaign to demand that voters are given a voice on the final Brexit deal.
Sign our petition here
