Gordon Brown will make a high-profile intervention in the Labour leadership contest as the rival candidates stepped up their efforts to win support.
The former prime minister, who is held in high regard within the party and was credited with helping to turn the tide in the Scottish referendum campaign, will deliver an address on “power for a purpose”.
His speech comes as Andy Burnham insisted that only he was capable of blocking front-runner Jeremy Corbyn and preventing a damaging split within the party.
With more ballot papers set to be delivered in the coming days the shadow health secretary claimed only he could stop the left-winger and avert a period of 1980s-style infighting.
But in a sign of the left-winger’s polarising effect, the poll also found he was the contender rated as having the best chance of improving Labour’s fortunes.
Some 21% of those surveyed by ComRes thought Mr Corbyn would boost Labour’s chances at the next election, putting him ahead of Mr Burnham on 19%, Yvette Cooper on 15% and Liz Kendall on 11%.
But the study, for the Independent on Sunday and Sunday Mirror, found 31% of those polled thought Mr Corbyn would worsen Labour’s prospects of electoral success - with Ms Cooper on 18%, Ms Kendall on 17% and Mr Burnham on 14%.
The figures gave Mr Burnham a net rating of +5, with Ms Cooper on -3, Ms Kendall on -6 and Mr Corbyn on -10.
None of the candidates performed as well as former foreign secretary David Miliband, defeated by his brother Ed at the last Labour leadership contest - who scored a net rating of +11.
Among Labour voters, some 33% thought Mr Corbyn would improve the party’s chances of success at the next election, with 21% saying he would worsen them.
Mr Burnham told the Sunday People: “I’m the only person in this race who can beat Jeremy.”
He added: “In the 80s, we started fighting each other and left the way clear for Margaret Thatcher to bulldoze her way through Labour communities.
“I’m not going to let that happen this time.”
In an indication of the task facing whoever emerges as the winner of the leadership race on September 12, the ComRes study found the Opposition trailing the Tories by 11 points, with David Cameron’s party on 40% and Labour on 29%.
Meanwhile Ms Cooper launched plans to hit payday lenders with a levy in order to fund the expansion of credit unions.
In a Sunday Mirror interview she said: “Too many of these payday lenders are still ripping people off and profiting from desperation.
“It’s time those lenders paid for an affordable alternative. That’s why I think they should pay to support credit unions that can help people instead.”
Mr Corbyn sought to calm fears that he would have an anti-business agenda by setting out plans to support entrepreneurs and small traders.
He told The Observer: “The current government seems to think ‘pro-business’ means giving a green light to corporate tax avoiders and private monopolies. I will stand up for small businesses, independent entrepreneurs, and the growing number of enterprises that want to cooperate and innovate for the public good.
“My Better Business plan will level the playing field between small businesses and their workers who are being made to wait in the queue behind the big corporate welfare lobby the Tories are funded by and obsessed with.”
Sources in Ms Cooper’s camp have played down reports that Mr Brown is set to use his speech to endorse her for the leadership, but the shadow home secretary admitted she has spoken to the former prime minister.
“I have spoken to Gordon, I have spoken to lots of different people as part of the campaign, just asking people about their ideas for the future and so on,” she said.