Jeremy Corbyn is braced for a showdown over Labour’s commitment to a referendum on any Brexit deal.
His party’s governing body the National Executive Committee will meet later on Tuesday to agree its European election manifesto.
The crunch talks will be aimed at finalising Labour’s position on the UK’s withdrawal from the EU for next month’s polls.
Pressure has been mounting on Mr Corbyn to push for a People’s Vote as cross-party talks to break the impasse continued.
De facto prime minister David Lidington described the latest discussions on Monday as "positive" and "productive".
EU's chief Brexit negotiator Michel Barnier said he hoped to see results this week from the talks.
He reiterated a call to "fix Brexit quickly".
Meanwhile Change UK - the Independent Group – is to hold the first of its "People's Vote Remain" rallies in London calling for a second referendum.
Ahead of the NEC meeting, Labour MPs Peter Kyle and Phil Wilson wrote to Mr Corbyn and all NEC members urging them to ensure that a confirmatory ballot is in the manifesto.
Mr Wilson said a referendum was backed by the majority of Labour MPs, MEPs, members and supporters and had featured in the policy approved by conference last autumn.
Labour MPs had twice been whipped in the Commons to support a motion tabled by the pair to require any Brexit deal to be subject to a public vote, he said.
"Three years on, we know more about what Brexit means and people should be allowed to compare what the deal is with what they were promised and decide whether they want to go ahead," said Mr Wilson.
"If we don't do it, the coming years will be divisive because people will be turning round and saying 'Hang on, I didn't vote for this'."
Some 115 MPs and MEPs signed a letter to NEC members organised by the Love Socialism, Hate Brexit group urging them to explicitly back a referendum in the manifesto.
Deputy Labour leader Tom Watson used his Twitter account to urge followers to lobby NEC members to back a public vote.
And 22 Labour candidates in the European elections have pledged to campaign for a referendum and then back efforts to remain in the EU
Meanwhile, some of Labour's biggest union backers are also throwing their weight behind a referendum pledge.
TSSA general secretary Manuel Cortes said: "Frankly it's time for our party to act on the overwhelming wishes of its members and voters by pledging to support a confirmatory public vote on any Brexit deal.
"We will be pushing hard for this to be included in Labour's European manifesto."
