Labour party member Jeremy Corbyn, a leftwinger touted to head Britain's Opposition, said on Friday he would apologise over the Iraq war if he wins next month's leadership contest.
"It is past time that Labour apologised to the British people for taking them into the Iraq war on the basis of deception and to the Iraqi people for the suffering we have helped cause," Corbyn said in a statement to the Guardian newspaper.
"Under our Labour, we will make this apology."
Under the former Labour prime minister Tony Blair, Britain joined the United States led by George W Bush in invading Iraq in 2003 and had troops there until 2011.
That decision is now deeply unpopular and has haunted the Labour party ever since.
Corbyn, who strongly opposed the Iraq war, is expected to sweep to victory when the results of the leadership contest are announced on September 12.
His anti-austerity views are well to the left of current Labour policy, leading to warnings from senior figures such as Blair that his leadership would make the party unable to win a general election.
At the time of the Iraq invasion, Blair advocated joining it by arguing that Saddam Hussein had weapons of mass destruction, but evidence has never been found.
The former prime minister argues it was still right to remove Saddam as he posed a threat to the region.
A YouGov opinion poll in June indicated that 51% Britons now think it was wrong to take military action in Iraq, compared to the 26% who think it was right.
Corbyn's announcement comes as pressure grows for an inquiry into Britain's role in the Iraq war to be released after six years.
The Chilcot inquiry is being delayed by a a process in which individuals named in the report are given the chance to respond to their criticism.
Elsewhere in his statement, Corbyn also stressed he would be cautious of military interventions overseas.
"Let us say we will never again unnecessarily put our troops under fire and our country's standing in the world at risk," Corbyn said.
Prime Minister David Cameron's government is considering asking Parliament to approve air strikes on Islamic State (IS) group targets in Syria.
But he would need Labour's support to get this through and Corbyn's stance indicates this would not be forthcoming if he became leader.