Labour’s deputy leader Tom Watson is to urge his party to “strongly” make the case for the UK staying in the EU.
In a speech later, he will reiterate his push for another referendum to break the Brexit deadlock – saying Labour members “hearts are Remain”.
Leader Jeremy Corbyn has so far resisted calls to fully back another public vote, saying the party would support it in certain circumstances.
That nuance was blamed for Labour’s performance at the European elections.
The party came third behind The Brexit Party and the Liberal Democrats, with its share of the vote falling to 14%.
Shadow foreign secretary Emily Thornberry said the results boosted the case for another referendum – an outcome Mr Corbyn has said is “some way off”.
Labour MPs remain split, with those representing Leave areas warning against backing a further public vote.
Last week, MPs expressed their frustration at a heated meeting of the Parliamentary Labour Party, with some expressing concern that it had become “normalised” for Labour voters to back other parties over Brexit.
The shadow cabinet was due to meet on Monday to discuss Brexit, but the meeting has been postponed.
‘Brexit not our future’
In a speech to the Centre for European Reform, Mr Watson – who has repeatedly put pressure on Mr Corbyn to back a further referendum – will say there is still time to reverse the Brexit vote.
“Pro-European is who we are and who we have always been. Our members are Remain. Our values are Remain. Our hearts are Remain,” he will say.
“The only way to break the political deadlock is to bring the public back into this decision and we must argue strongly to remain.
“Our future doesn’t need to be Brexit. We can change our future. We can put Britain back at the heart of Europe again.
“We can be proud of leading the fight for a fairer and stronger future, together. But we can only achieve this future if Labour fights for it and champions it. It’s time we do that.”
He received support from a number of colleagues, including Jess Phillips and Anna Turley.
Another MP, Siobhain McDonagh, tweeted: “I have had my differences with Tom Watson over the years but this video is brilliant and his argument is bang on! So many Labour members will be cheering him on!”
BBC political editor Laura Kuenssberg says plenty of Labour MPs are worried because they represent constituencies with Leave voters, but there is no question the balance in the party is on the other side.
“There are plenty of senior people – including those absolutely loyal to Jeremy Corbyn – who think it is time for the leadership to make a clearer statement arguing for another referendum and for Britain to stay in EU,” she says.
“Some of those think it is vital to do before the summer and they predict we may end up with an election in the autumn with the Tories arguing for Leave and Labour arguing for Remain.”
Labour chairman Ian Lavery – who is against another referendum – has previously called on pro-Remain members of his party to stop “sneering at ordinary people” who support Brexit.
However, Mr Watson will say that all strands of opinion within the party are entitled to be heard.
He will also argue that the “core” EU values of internationalism, solidarity and freedom are also the values of Labour.
“Some people have begun to equate support for Europe with class identity – I don’t think that’s right or helpful,” he will say.
“The majority of Labour people are supportive of Europe and that support is not dictated by social class.”