The secret file, compiled by Labour party aides for general secretary Iain McNicol, also found the former leader was never ahead of David Cameron in leadership polls.
It came despite Dame Margaret Beckett's report which found that other pressures, including Labour's failure to tackle its record on the economy, were to blame.
According to the Daily Mail, which has seen the new report, Labour lost the vote of the middle classes, was hit much harder by Ukip than it expected and Mr Miliband was not thought to be strong enough to withstand the demands of Nicola Sturgeon and the SNP.
Focus group participants said Labour came across as "downtrodden, in thrall to the undeserving, in denial about their appalling record – personified by Miliband".
In Dame Beckett's report the media was to blame for painting a negative picture of Mr Miliband, who was "courageous, well-regarded and thoughtful".
The secret dossier also found that Labour lost middle-income earners to the Conservatives, who offered a more aspirational promise, and was too focused on low earners.
The party was also dogged by former minister Liam Byrne's infamous "There's no money left" note, which symbolised the party's attitude to the public finances.
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