Both Ed Balls and Ed Miliband have criticised the Iraq war
Two of the candidates attempting to become the next Labour leader have criticised the decision to invade Iraq.
Ed Balls, the former children's secretary, said the war was a "mistake" and was "wrong".
And former energy secretary Ed Miliband said the way the decision to go to war was taken had caused "a big loss of trust" for the Labour party.
Rival candidate John McDonnell said he welcomed their "road to Damascus conversion" but it was far too late.
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.
He called on the two to join him in a call for the immediate withdrawal of troops from Afghanistan.
He said: "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."
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."
Neither Mr Balls or Mr Miliband were MPs when the decision to invade Iraq was made.
Rose Gentle, whose 19-year-old son Gordon, was killed in 2004 while serving in Iraq, said she could not understand the timing of their statements.
She said: "It's an insult to the families just now."
The result of Labour's leadership contest will be known on 25 September.
The other candidates for leadership are Diane Abbott, Andy Burnham and Ed Miliband's brother, David - who was an MP at the time of the invasion of Iraq and voted for it.
Mr Balls told The Daily Telegraph: "We shouldn't have changed our argument from international law to regime change in a non-transparent way. It was an error for which we as a country paid a heavy price, and for which many people paid with their lives."
He later told the BBC: "There wasn't the evidence to justify going to war and, in retrospect, we shouldn't have done it and I think it's very important that we say that."
Mr Balls said he would not be calling for an immediate withdrawal of troops in Afghanistan.
Ed Miliband told the Guardian the basis for going to war was Saddam Hussein's potential possession of weapons of mass destruction.
He said UN weapons inspectors should have been given more time to find out whether he had those weapons.
"The combination of not giving the weapons inspectors more time, and then the weapons not being found, I think for a lot of people it led to a catastrophic loss of trust for us and we do need to draw a line under it."
He added: "What I am not saying is that the war was undertaken for the wrong motives but what I am very clear about is what my position was at the time and the way I look at it in retrospect."
Mr Miliband's brother, David, said the decision to invade had been "damaging" to the Labour Party but his position on Iraq had remained consistent.
"I voted for the war in Iraq to uphold UN resolutions about weapons of mass destruction. As I said to [the Chilcott Inquiry into the Iraq war] in answer to the question about whether I would have voted for the war if we had known there were no weapons of mass destruction - no," he said.
Ms Abbot said she believed the Iraq war would be an important issue for the candidates to address.
She said it was "commonly understood that Iraq was the single biggest cause of disillusion amongst Labour activists".
Former Cabinet Office minister Tessa Jowell, who voted for the invasion and is not one of those standing, said it was important such issues were discussed by the candidates and that there were no "no-go areas" when it came to subjects which defined the Labour government.
The Miliband brothers and fellow candidate Andy Burnham are set to speak at left-wing think tank Progress's annual conference in London later.