David Miliband acknowledged Iraq had been a "source of division"
Former foreign secretary and Labour leadership contender David Miliband has urged party members not to make the Iraq war an issue in the race.
He said it was "time to move on" after two other candidates criticised the decision to invade Iraq in 2003.
Ed Balls said the war was "wrong", while Ed Miliband said it had led to a "big loss of trust" in Labour.
Rival candidate John McDonnell said their "road to Damascus conversion" was far too late and opportunistic.
Mr McDonnell, MP for Hayes and Harlington, who opposed the war, said many lives might have been saved had the men had the "courage of their convictions" to speak out at the time.
"Others have said [their comment] smacks of opportunism because of the leadership election but I want to say to them is we have got another war now and it's Afghanistan," he said.
'Come to terms'
Neither Mr Balls, the former children's secretary, or Mr Miliband, the former energy secretary and younger brother of David Miliband, were MPs when the decision to invade Iraq was made.
In separate newspaper interviews both men sought to distance themselves from the conflict.
Both Ed Balls and Ed Miliband have criticised the Iraq war
Mr Balls, who was an adviser to Gordon Brown in the Treasury at time of the invasion, acknowledged he would have voted for it had he been an MP but said it would have been the wrong thing to do.
"It's very important that we say as a Labour party, to rebuild trust, on those areas where we got it wrong, where we made a mistake, that we say that," he said.
"And I think it's important that we say that on Iraq."
Ed Miliband said he had spoken out now because it was important for Labour to acknowledge why it had lost power.
"I think there's no question that, for the Labour party and the Labour government, we went to war on a particular basis which turned out not to be vindicated - in terms of weapons of mass destruction," he told the BBC.
"There's no doubt that for a lot of people that caused a big loss of trust for us."
'Source of division'
David Miliband, tipped as the frontrunner to succeed Mr Brown, was an MP at the time and voted for the invasion.
BBC political correspondent,
Progress conference, London
The invasion in 2003 is something three of the candidates want to talk about.
Ed Miliband was not a MP and not in government at the time. Neither was Ed Balls - although he was a senior advisor to then chancellor Gordon Brown.
This is an attempt to distance themselves from an issue so closely associated with the Labour government - and one that remains divisive within the party.
It could also be seen as an attack on David Miliband - a former foreign secretary and a member of the government at the time who supported the invasion. He's admitted the decision to invade 'damaged' Labour.
Left-wing contender John McDonnell has accused his rivals of pure opportunism.
Labour is still learning to be in opposition.
At this conference the catharsis has only just begun but some clearly still believe Iraq hurt them this time round.
He give his response to his rivals' comments as he arrived at the annual conference of the left-wing think tank, Progress, in London.
"While Iraq was a source of division in the past, it doesn't need to be a source of division in the future," he said.
"Iraq was a big issue at the 2005 general election but the vast majority of MPs and candidates I have spoken to this time say that while it was a big issue then it was much less of an issue in 2010.
"I said during the election campaign that I thought it was time to move on."
He repeated the assertion that, had he known there were no weapons of mass destruction in Iraq, he would not have voted for the war.
The other candidates for the leadership are Diane Abbott and Andy Burnham.
Mr Burnham, the former health secretary who voted for the war, told the BBC he did not believe it would be a major issue in the leadership contest.
"Personally, I don't back away from the decision that we made," he said.
"I still believe that it was right to remove Saddam so that Iraqis could have a hope of a better future and a better life."
But Ms Abbott, MP for Hackney North and Stoke Newington, disagrees.
She said it was "commonly understood that Iraq was the single biggest cause of disillusion amongst Labour activists".
The Miliband brothers and Mr Burnham all spoke at the Progress annual conference on Saturday, setting out their vision for the future of the Labour party.
David Miliband said New Labour had created a model of leadership based on "discipline not dialogue" that was now in need of fundamental change.
While Mr Burnham told the conference the party had suffered because, in seeking to show it was not opposed to aspiration and wealth creation, it had gone too far.
The result of Labour's leadership contest will be known on 25 September.