Labour has "rediscovered losing", former Prime Minister Tony Blair warned as a poll put left-winger Jeremy Corbyn ahead in the leadership contest.
Mr Blair said Labour could win again - but not from a "traditional leftist platform" and said it had to "move on".
Mr Corbyn said it was "a bit premature" to talk about him winning.
The YouGov poll for the Times suggests that in the final round of voting, the Islington North MP would finish six points ahead of Andy Burnham.
It shows Mr Corbyn as the first preference for 43% of party supporters, ahead of Mr Burnham on 26%, Yvette Cooper on 20% and Liz Kendall on 11%.
Taking second preferences into account the poll, of 1,054 people eligible to vote in the contest and carried out between Friday and Tuesday, pointed to a 6% Corbyn victory.
'Heart transplant'
Yvette Cooper and Liz Kendall are the other candidates for the leadership.
Mr Corbyn said he did not want to discuss opinion polls but said his campaign was going "extremely well".
"A lot of people are supporting us, particularly young people who want to see a Labour Party that is very different to the Labour Party they have had in the past," he added.
One of Mr Corbyn's supporters, former London mayor Ken Livingstone, said he had "got the economic strategy right" and that it was "waffle" to suggest it would divide the party if he won.
But Yvette Cooper said the party would not win by either moving sharply to the left or right.
"We do have to stand up for the ideas we believe in but we have to make sure that we can deliver," she said. "Otherwise it is people who depend on Labour who are going to be let down by this. That is why it is so important and serious that we are credible".