Vince Cable urges Barnier to prepare for new referendum
The EU's chief Brexit negotiator has been told to begin "serious contingency planning" for another EU referendum.
Lib Dem leader Sir Vince Cable said MPs backing a referendum on the final deal negotiated were becoming "the biggest and most cohesive bloc in Westminster".
Sir Vince, the SNP's Ian Blackford, Plaid Cymru's Liz Saville Roberts and Green MEP Molly Scott Cato met Michel Barnier in Brussels.
The PM has ruled out a referendum on the outcome of the Brexit negotiations.
But her "Chequers plan" - which would keep the UK closely aligned with the EU in trade in goods - has been heavily criticised by Tory Brexiteers, including Boris Johnson.
Mr Barnier, who has been negotiating on behalf of the other 27 EU members, has met other politicians from different parties already - including Labour Leader Jeremy Corbyn and former UKIP leader Nigel Farage.
In a joint statement after the hour-long meeting, the four parties said the "current Brexit trajectory" enjoys no majority in Parliament and accused Labour - the official opposition party - of having "waved through" the "destructive Brexit being pursued by the Tories".
"With sensible politicians from all parties uniting, we pointed out to Mr Barnier that there is a genuine cross-party consensus that our exit from the European Union must not be assumed," they said.
Sir Vince said: "My message to Michel Barnier was clear: it's time to start serious contingency planning for a 'people's vote'. We know the UK government has started making such plans as a result of the growing demand for such a vote, demonstrated by last weekend's march.
"The EU should do the same, because MPs who back the people's vote are fast forming the biggest and most cohesive bloc in Westminster."
The People's Vote campaign group wants to give the public the final say over whether the UK leaves the EU, arguing that voters should be given a choice between leaving with or without a deal or staying on current terms.
Hundreds of thousands of people took part in a march urging a referendum on the final Brexit deal at the weekend.
While Labour leader Jeremy Corbyn has previously ruled out another EU referendum, Labour members voted to keep the option on the table, if Parliament is deadlocked over the final outcome of the Brexit negotiations. Mr Corbyn has said he will respect the result of the vote.
And a handful of Tory backbenchers, including Heidi Allen and Anna Soubry, have said they would support another referendum.
But Prime Minister Theresa May has said a referendum on any withdrawal agreement that is agreed would be a "gross betrayal of our democracy" and trust.
The British public voted to leave the EU by a margin of 51.89% to 48.11% in a referendum in June 2016.
The UK is scheduled to leave on 29 March 2019, under the terms of the two-year Article 50 process.