Jeremy Corbyn could hold the key to Britain remaining in or leaving the European Union, according to new polling that shows the vote is on a knife edge, with the result potentially hanging on the decision and turnout of Labour voters.

The Labour leader is by far the most influential figure in the debate for his party’s voters, according to the poll by GQRR, commissioned by the Fabian Society.

Nearly two thirds of Labour voters say they are likely to vote to Remain – around six million people. However, little more than half of them say they are very likely to turnout to vote. Researchers said Mr Corbyn’s “full-throated support” for a Remain vote could be crucial in getting Labour voters to the polls and swinging the vote.

The new poll also suggests that the arguments for leaving the EU are proving more decisive among voters. While initial voting intentions give the Remain camp a 45 – 40 lead over Leave, when the two campaigns’ main arguments were put to voters, Leave surges ahead, tying the vote at 42-42.

Union leaders warned that David Cameron and the Remain campaign were at risk of losing the referendum and called on the Prime Minister to change gear and focus on arguments for staying in the EU aimed at “ordinary British workers” rather than “the boardroom and the City”.

The findings will also pile pressure on Mr Corbyn to take a more strident pro-EU stance.

While officially backing a Remain vote, Mr Corbyn was ambivalent about the EU during his leadership campaign and was has been a vocal Eurosceptic in the past. He has so far maintained a low profile in the EU referendum debate.

Corbyn on the EU

However, the Fabian Society poll finds that he is the most trusted figure in the debate for Labour voters, with a net approval rating of +17, well ahead of the party’s official campaign chief Alan Johnson, on -10.

The polling report’s author, Olivia Bailey, research director at the Fabian Society, said the Labour party was not doing enough to galvanise its voters ahead of the June referendum.

“Jeremy Corbyn’s full-throated support is vital for Labour remainers, as is the strength of Labour’s electoral machine,” she said. “But there are few signs of either whirring into action.”

In another worrying indicator for the Remain campaign, the poll, which surveyed more than 2,200 people, found that Leave voters tend to be more loyal to the cause and are more likely to turn out. Among those voters considered very likely to go the polls, Leave has a 47-45 lead.

Andrew Harrop, general secretary of the Fabian Society, said the findings should be a “wake-up call” for Remain campaigners.

“Leave's arguments appear to have more power to persuade than the arguments being used by Remain, and their voters are more motivated to vote. Without action from Remain, the EU race could shift in Leave's favour,” he said.

Overall, immigration and control of our laws are the biggest issues in the EU debate for voters, but among Labour voters, the security of British jobs emerges as the main concern, the poll found.

Frances O’Grady, general secretary of the TUC said: “This research shows that ordinary British workers will be decisive in the European referendum – and campaigners should focus on workers’ rights and jobs.

“But this will come as news to the Prime Minister, who’s running a Remain campaign focused only on the boardroom and the City. He might just lose this referendum if he doesn’t start telling working people what’s in it for them.

“This report also poses a challenge to Leave campaigners. They must come clean with workers that Brexit will mean the loss of European guarantees for important rights and protections at work.”

Responding to the poll, Alan Johnson, leader of the Labour In For Britain campaign, denied his party were “complacent” about the referendum.

“Unlike the Tories who are divided on this issue, Labour are united in our campaign for Britain to remain in Europe,” he said.