Miliband is moving to take control over the Labor Party. Labor's former Prime Minister Gordon Brown lost his leadership position to Conservative leader David Cameron in elections in early May.
Miliband acknowledged that the decision to go to war with Iraq was divisive but said it was time to move on, the BBC reports.
"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.
Miliband backed the invasion when he was in Parliament in 2003.
John McDonnell, who is challenging Miliband and others for control over Labor, labeled his rivals as opportunists.
McDonnell, an opponent of the Iraq war, said countless lives would have been saved if his rivals had "courage of their convictions" during debates over the 2003 invasion, the BBC added.
London, meanwhile, is examining its role in the Iraq war from the planning stages shortly after the Sept.11, 2001, attacks on the United States to the withdrawal of British forces in 2009. A total of 179 British service members died during operations in Iraq.