Labour’s deputy leader Tom Watson today warns that his party will lose the next general election and any chance to form a radical, reforming government unless it quickly rewrites its Brexit policy and commits wholeheartedly to a second referendum.

Watson’s intervention comes after the shadow chancellor, John McDonnell, appeared to suggest a shift in Labour’s position towards stronger support for a second public vote.

Writing in Sunday’s Observer, before tonight’s European election results, which he fears will show Remain voters have deserted to the Liberal Democrats and other pro-EU parties, Watson says Labour must develop “backbone” on Brexit as a matter of urgency. It has to end its “mealy-mouthed” backing for another public vote if it is to be in tune with its members, he writes.

He also pledges to back a growing movement within the party which is demanding that Brexit policy is changed before the autumn party conference. This will give Labour enough time to campaign for a referendum with Remain on the ballot paper, before the UK is due to leave the EU at the end of October.

Watson says Labour has been far too timid and unclear, infuriating its Remain voters to such an extent that lifelong supporters have deserted. “Once results are in, we must channel our frustration into preventing this mistake repeating itself and winning those voters back,” he says.

“Never again can Labour policy on the most crucial issue of our generation find itself on the wrong side of its members and our voters.

“Never again can we find ourselves hedging our bets when we needed to make an historic choice about which side we’re on.”

He adds: “ I want the reports of high turnout in Remain areas to reflect a huge gain in votes for Labour, but I fear that won’t be the case,” Watson writes. “Pessimistic predictions put Labour in third place in this European election behind both Nigel Farage’s far-right party and the born-again Liberal Democrats. Even optimists have us in second place and nowhere near the percentage of votes we would need to win a general election.

“This was the first election I can remember where Labour party members, not voters but party members, said they couldn’t support us. One member I met in Bristol broke down in tears as he told me that after 44 years of voting Labour he was going to vote Lib Dem for the first time. Our performance is a direct result of our mealy-mouthed backing for a public vote on Brexit when it is being demanded loud and clear by the overwhelming majority of our members and voters.”

Speaking on BBC Radio 4’s Today programme, McDonnell said it was time for a fresh “conversation” about the way forward on Brexit. “I can’t see any other way but demanding a public vote of some form, whether it’s a general election or some other public vote. We’ve got to stop a hard Brexit situation,” he said.

He added: “It looks almost certain we’re going to be faced with a Conservative leader who is a hard Brexiteer willing to take the country over the edge of a no-deal no matter what the damage to jobs or people’s livelihoods.

“Faced with that situation, I think there may well be a majority in the House of Commons willing to bring about some form of public vote and that could include a general election.”

Watson says polls suggest Labour has been losing up to four times more voters to parties giving full backing to another referendum, than to Nigel Farage’s Brexit party. He adds: “I fear that unless our policy on Brexit changes we will not have the opportunity to be the radical reforming government that so many millions of people in our country need. The campaign to change that begins now.”

Senior Labour figures are braced for a bitter backlash from pro-EU supporters if tonight’s results confirm their worst fears. Despite pressure from Remainers in the party for a clear commitment to a second referendum to be included in Labour’s European election manifesto, the ruling national executive committee (NEC) approved wording that merely confirmed a second public vote would be an “option” if the party could not secure a general election or a satisfactory Brexit deal.