The report, written by former Cabinet minister Dame Margaret Beckett, said the comparative standing of the two men was one of four reasons for the defeat.
Other factors identified were failures in economic and immigration policy and the fear of the SNP propping up Labour.
The report said Labour faced a "huge challenge" to win again in 2020.
The report concludes that the party needs a massive upswing in its fortunes in England and Scotland if it is to be returned to power in five years time.
It urges the party to do more to appeal to older voters and to construct a clear "policy narrative" that can be sold to the public.
The BBC's assistant political editor Norman Smith said the analysis made "sobering reading" for Labour, with weaknesses identified in key areas.
The report concluded that Labour's "ground campaign" during the election was strong but suggests many voters had already made their minds up and opinion polls suggesting it was neck and neck with the Conservatives were wrong.
Pollsters and activists on the doorstep consistently heard four reasons for the party's defeat, which saw it lose ground in key marginal seats in England and found itself all but wiped out in Scotland. They were:
On the issue of leadership, the report argues that Mr Miliband enjoyed a "surge" in 2015 and performed well during the campaign itself.
But it says he was unable to shift public perceptions, borne out in opinion polls, that David Cameron was regarded as more "prime ministerial".
"Over the period 2010-15, what the polls did consistently show was that, when asked if 'this man could be prime minister', David Cameron was rated above Ed Miliband. Since he actually was prime minister, this response was perhaps less than surprising.
"It is the fate of every Labour leader of the opposition to be the target of ferocious attack from partisans sections of the media," it added. "However, Ed Miliband faced an exceptionally vitriolic and personal attack."
The report says assumptions that Labour lost the election because it was too left-wing and because its policies were unpopular was overly simplistic and should be treated with caution, pointing out that the party's most radical initiatives - such as an energy price freeze and mansion tax on expensive properties - were among those most liked by the public.
But it says Labour lacked a consistent over-arching theme and struggled to communicate its approach to the voters in contrast to the "brutal simplicity of the Tories and UKIP".
"The reaction to the 2015 result was inevitably an emotional one for Labour because it was such a surprise," Dame Margaret said.
"There was certainly no complacency in the Labour ranks, but the polls showed us neck and neck with the Tories, when clearly we weren't.
"There are certainly lessons to learn from defeat. This report has been a key part of recognising areas we need to improve on and building on aspects of our campaign that performed well."
On the challenge facing Labour in 2020, the report concluded: "It is critically important for the country that Labour is able to win the 2020 election but we will face huge challenges. On current boundaries we need 94 gains to secure a majority of two.
"Were Labour to fail to recover in Scotland, it has been estimated that the swing required in England and Wales from the Tories to Labour would be 12.5% to secure an overall majority."
Shadow business secretary Angela Eagle said the party must pay close attention to the report as it sought to "remake its appeal", telling the BBC that Labour had come back from heavy defeats before and would do again.
But Tom Baldwin, Mr Miliband's former director of communications, said the current leadership had not given enough thought to why Labour lost. "Unless we start thinking about why we lost, we won't be able to win again," he told the BBC News Channel.
Ed Miliband was not judged to be as strong a leader as David Cameron, an internal Labour report into why the party lost the 2015 election has found.