Gordon Brown is to intervene in Labour’s leadership race for the first time, saying the party needs credible economic policies to win power.
He is also expected to say he strongly disagrees with many of Jeremy Corbyn’s economic and foreign policies.
His comments come as a poll suggests Mr Corbyn is considered the candidate most likely to worsen Labour’s prospects of winning the next election.
The other leader candidates are Yvette Cooper, Liz Kendall and Andy Burnham.
Voting has now opened in the contest to replace former party leader Ed Miliband.
Mr Brown, the former prime minister, will deliver an address on “power for a purpose”.
His speech comes as Mr Burnham claimed he was the only candidate capable of blocking Mr Corbyn, currently seen at the front runner, and preventing a damaging split within the party.
A ComRes study of 2,035 adults in Britain, for the Independent on Sunday and Sunday Mirror, found 31% of those polled thought Mr Corbyn would worsen Labour’s prospects of electoral success.
Ms Cooper was rated as 18%, Ms Kendall as 17% and Mr Burnham as 14% – making him the poll’s least damaging candidate.
The figures gave Mr Burnham a net rating of +5, Ms Cooper -3, Ms Kendall -6 and Mr Corbyn -10.
However, 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.
And of those surveyed, 21% thought Mr Corbyn would boost Labour’s chances of winning the next election, while Mr Burnham polled 19%, Ms Cooper 15% and Ms Kendall 11%.
Labour leadership contest
- Who are the candidates? Andy Burnham, Yvette Cooper, Jeremy Corbyn, Liz Kendall
- Dates: Ballot papers were sent out on 14 August; voting can take place by post or online. They must be returned by 10 September. The result is on 12 September
- Who can vote? All party members, registered supporters and affiliated supporters – including those joining via a union
- What is the voting system? The Alternative Vote system is being used so voters are asked to rank candidates in order of preference
- How does it work? If no candidate gets 50% of all votes cast, the candidate in fourth place is eliminated. Their second preference votes are then redistributed among the remaining three. If there is still no winner, the third place candidate is eliminated with their second preferences (or third in the case of votes transferred from the fourth place candidates) redistributed. It is then a head-to-head between the last two candidates
Mr Burnham told the Sunday People: “I’m the only person in this race who can beat Jeremy.
“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.”
The study also put support for the Conservatives at 40% and Labour at 29%.
The BBC’s political correspondent Carole Walker said Mr Brown would not personally criticise Mr Corbyn, but will make it clear he strongly disagrees with many of his policies, and does not believe he could lead Labour back into government.
It is unclear whether the former prime minister will declare his support for Ms Cooper – though it is understood he believes she is the best candidate to provide the leadership the party needs, our correspondent added.
Labour leadership poll
total electorate, though this may fall as party removes those not entitled to vote
Of which, full party members: 299,755
Affiliated to a trade union: 189,703
Registered to vote by paying £3: 121,295
Meanwhile, Mr Corbyn has moved to dampen concerns 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.”
Meanwhile, Ms Cooper has criticised some of Mr Corbyn’s anti-austerity policies, telling the Sunday Mirror that “I don’t think the answer is what Jeremy has proposed, which is basically printing money that we haven’t got to build things.”
She also told the paper that payday lenders must pay a levy in order to fund the expansion of credit unions.
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.”