Despite multiple interventions in the EU debate, Jeremy Corbyn must do more to engage voters, a top union boss has warned.
The GMB union's Tim Roache said the Remain campaign could end up losing the EU referendum if Labour voters aren't move involved in the debate.
While phone polls put Remain steadily ahead, online polling suggests a dead heat between Remain and Leave ahead of the vote in June, with concerns still lingering over turnout on the Remain side.
Roache's comments came ahead of the Labour leader's planned speech on the EU, where was expected to warn that workers' rights are protected by the EU.
Read more: Corbyn issues Tory Brexit warning
In an interview with the BBC, Roache said he was extremely concerned about Labour voters simply staying at home instead of getting to a polling booth, adding that in his view, Corbyn has been a "half-hearted" Remainer.
"I think he probably could do a bit more, let's hope he will," he said.
"I think they won't see it as the absolutely crucial vote that it is and I also think that they see it as a bunfight in the Tory party," he added.
"The reality is that the more people that stay at home the more likely it is that we will leave the EU. I don't think we can even begin to contemplate that."
Corbyn has already spoken on the EU a number of times, but some have complained he has not argued passionately enough for the merits of membership.
Today Corbyn will warn that a vote for Brexit would allow the Conservatives a strong hand in reducing workers' rights.
Shadow chancellor John McDonnell has also spoken of the risks of a Tory Brexit, as Labour leadership has been keen to separate itself from the Conservative campaign.
Labour was punished in Scotland after the independence referendum after having campaigned alongside Tories. However, last weekend Mayor of London Sadiq Khan campaigned with Prime Minister David Cameron, while chancellor George Osborne has previously been joined by his former opponent Ed Balls.
Most recently Osborne wrote a joint letter with former chancellor Alistair Darling to slam Vote Leave for what they termed "uncosted and unworkable proposals" on the economic consequences of leaving the EU.
"If you can't answer these straightforward questions you are confirming that a vote to leave is a leap into the dark," they wrote.