Almost two-thirds of Labour supporters (65%) think David Miliband would be a better leader for the party than his brother Ed, according to private polling released by the Conservative Party.
The figure is the most devastating finding in a poll which suggests Ed Miliband is facing difficulties persuading voters that he is a potential prime minister.
The Tories released the findings from the survey of 2,000 voters in an apparent attempt to undermine Mr Miliband's position as Labour delegates gathered for the party's annual conference in Manchester.
Conservative strategists believe the survey reveals doubts about Mr Miliband's leadership which may mean that Labour will not be able to translate its consistent double-digit lead in the polls into actual votes when people go to the ballot box.
Its release appears to indicate that Tories are pinning their hopes on casting the election as a choice - which they are confident they can win - between Mr Miliband and David Cameron as the best national leader.
Almost three-quarters (73%) of those questioned by pollsters Populus agreed that Mr Miliband did not have what it takes to be prime minister in tough economic times, and was too weak to be a credible leader (72%), Tory sources said. Some 67% said Labour chose the wrong Miliband brother and 51% said they would be more likely to vote Labour if the party had a stronger leader.
Worryingly for Mr Miliband, large numbers of swing voters, who said they could switch their party allegiance between now and the election, agreed with these statements - as did around half of those who said they were planning to vote Labour. And 65% of those identifying themselves as Labour supporters said David Miliband would be a better leader than Ed.
Some 80% of swing voters and 46% of Labour backers said Ed Miliband did not have what it takes to be PM in hard economic times; 79% of swing voters and 47% of Labour supporters agreed he was too weak to be credible; 67% of swing voters and 56% of Labour supporters said the party chose the wrong brother in 2010; and 64% of swing voters and 59% of Labour backers said they would be more likely to vote Labour under a stronger leader.
Mr Miliband also scored poorly on a range of character issues which are often seen as a guide to how voters view potential prime ministers. Crucially, only 20% regarded him as "prime ministerial", compared with an overwhelming 80% who did not. Only 23% thought he was "the best leader his party has available", while 77% said he was not. Even among Labour voters, just 38% said Mr Miliband was the best leader their party had available and only 35% said he would be a strong national leader.
Among all those questioned, just 36% agreed he was "strong and determined", against 64% who disagreed. He was viewed as "charismatic" by 18% and not by 82%. Some 17% saw him as "a natural leader", while 83% did not. Some 19% said Mr Miliband was "respected by international leaders", compared with 81% who thought he was not; 25% said he could be trusted to repair the UK's finances, against 75% who said he could not.