Mr Miliband says Labour's problems got worse under Gordon Brown.
David Miliband has criticised Gordon Brown's record in office, saying Labour's failings worsened under him.
Mr Miliband said the former Prime Minister had failed to demonstrate the "moral seriousness" he had promised.
The shadow Foreign Secretary said that by the time of the election in May Labour had lost the people's trust.
Mr Miliband, who is running to succeed Mr Brown as party leader, was giving a lecture in memory of Labour founder Keir Hardie in south Wales.
Mr Miliband told the audience in Mountain Ash, Cynon Valley, that he had completely agreed with Mr Brown when he took over from Tony Blair.
Pledges failed to materialise
He said: "I supported and voted for him. I agreed that we needed greater moral seriousness and less indifference to the excesses of a celebrity-drenched culture.
"I agreed with him when he said that we needed greater coherence as a government, particularly in relation to child poverty and equality.
"I agreed with him on the importance of party reform and a meaningful internationalism that would be part of a unified government strategy.
"I agreed that we needed a civic morality to champion civility when confronting a widespread indifference to others.
"But it didn't happen."
Mr Miliband was publicly loyal to Mr Brown when he was foreign secretary to the then prime minister, although many observers suspected he harboured leadership ambitions.
In his speech he said Labour's problems had got worse after Mr Brown became leader.
By the time of May's general election Labour had "lost the trust of the people - and in a democracy that's a very big problem," Mr Miliband said.
"It was not just more of the same. Far from correcting them, failings - tactics, spin, high-handedness - intensified, and we lost many of our strengths - optimism born of clear strategy, bold plans for change and reform, a compelling articulation of aspiration and hope.
"We did not succeed in renewing ourselves in office - and the roots of that failure were deep not recent, about procedure and openness, or lack of it, as much as policy," he added.