Tom Watson has renewed calls for Labour to back a second referendum, saying "our future doesn't need to be Brexit".
The party's deputy leader 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.
"Socialism is achieving common causes by the strength of collective endeavour," he said.
"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."
Jeremy Corbyn has been resisting pressure to fully embrace a second referendum, amid heavy criticism of the party's poor results in last month's European elections.
However Mr Watson said Parliament "must bring the public back into this decision"
"Our future doesn't need to be Brexit. We can change the future. We can put Britain back at the heart of Europe again," he said.
"We can be proud of leading the fight for a fairer and stronger future, together."
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 Labour MP Steve McCabe has said that “Labour have got to offer a much clearer choice” on Brexit.
He told talkRADIO’s Mike Graham: “I think what Tom Watson is saying is that if we are going to end up having a Boris Johnson premiership then Labour have got to offer a much clearer choice between what Boris would do and what Labour could do."