Nearly 90 Labour MPs and MEPs have demanded the party officially backs another referendum.
Labour's National Executive Committee (NEC) has been urged to throw its weight behind a public vote on any Brexit deal in the European manifesto.
A letter from 75 MPs and 14 MEPs sets out plans to beat Nigel Farage's Brexit Party with "a message of hope and solidarity", ahead of a key NEC meeting on Tuesday.
"Labour has already, rightly, backed a confirmatory public vote," the letter states.
"The overwhelming majority of our members and voters support this, and it is the democratically established policy of the party.
"We need a message of hope and solidarity, and we need to campaign for it without caveats."
Labour grassroots were outraged by a leaked draft of a campaign leaflet on Thursday, which made no mention of another referendum.
Instead, the mailshot pledges a Labour government would seek "a better deal with Europe" without mentioning the possibility that Brexit may still not happen.
Now, the Love Socialism, Hate Brexit group of MPs has decided to act, stressing the polls are clear that this election is a two-horse race between Labour and Nigel Farage's Brexit Party.
Polls also show the overwhelming majority of Labour's members, supporters and potential voters want a public vote on any Brexit deal with an option to Remain, the group said, and failing to offer this risked the future of the Labour movement.
"To motivate our supporters, and to do the right thing by our members and our policy, a clear commitment to a confirmatory public vote on any Brexit deal must be part of our European election manifesto," the letter states.
"We understand the many different pressures and views within our movement, but without this clear commitment, we fear that our electoral coalition could fall apart."
