Labour deputy leader Tom Watson has launched a fresh push for the party to fully commit to a second referendum in an effort to stay in the European Union.
Mr Watson said “our future doesn’t need to be Brexit” as he argued that the European Union’s values were shared by Labour.
Jeremy Corbyn has been resisting pressure to fully embrace a second referendum, amid heavy criticism of the party’s dismal showing in last month’s European elections.
Confusion over its position as supportive of another referendum in certain limited circumstances, was widely blamed for a poor performance which saw it beaten into third place behind the Brexit Party and the Liberal Democrats.
Mr Watson said the prospect of Tory leadership frontrunner Boris Johnson becoming prime minister meant there was an urgent need to act.
He used a video message to set out his case, claiming the left had been too timid to defend European values.
“I love Europe because I’m a democratic socialist,” he said.
“Socialism is achieving common causes by the strength of collective endeavour.
“That’s what Europe is. We’ve shied away from saying these things for too long.
“But now, as we stare down the barrel of a Boris Johnson premiership, we really must.”
He said the relationship with Europe was about “more than economics” or political co-operation.
“It’s about what kind of country we are. What we want for our children: what we’re able to bring them up to be.
“So those of us who love Europe should take pride in making this argument.
“We must 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 the 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.”
Mr Watson will set out further details of his position in a keynote speech to the Centre for European Reform,.
He will argue that a referendum is “the only way to break the political deadlock” at Westminster.
Mr Watson is likely, however, to face a strong push-back from allies of Mr Corbyn, determined to resist any change of policy.
Last week party chairman Ian Lavery hit out at Remainers in the party, dismissing them as “left wing intellectuals” who were “sneering at ordinary people” in traditional Labour heartlands who voted for Leave.
However Mr Watson will say that all strands of of opinion within the party were entitled to a hearing.
“We cannot go on dismissing one another’s right to speak and questioning one another’s motives and intentions,” he will say.
“Some people have begun to equate support for Europe with class identity, I don’t think that’s right or helpful.
“The majority of Labour people are supportive of Europe – and that support is not dictated by social class.”