The Iraq War was thrust into the centre of the Labour leadership race after Jeremy Corbyn said he would apologise for the conflict and defended comments comparing Islamic State (IS) to the US military.
The surprise frontrunner said if he wins the contest he will issue a formal apology on behalf of Labour which led the country into the 2003 invasion of Iraq and subsequent conflict under Tony Blair.
Mr Corbyn, who fiercely opposed the war at the time, said Mr Blair had taken Britain into the conflict "on the basis of deception" and that his decision to support then-US president George W Bush in a joint invasion had cost Labour millions of voters.
In a statement to the Guardian, Mr Corbyn said: "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. Under our Labour, we will make this apology."
He went on: "It has also lost Labour the votes of millions of our natural supporters, who marched and protested against the war.
"We turned our backs on them and many of them have either withheld their votes from us or felt disillusioned, unenthusiastic and unmotivated."
Mr Corbyn also suggested that military interventions would be rarer if he had control of Labour, given the convention of governments seeking parliamentary approval before taking action.
His comments could prove significant if he wins as David Cameron is widely expected to ask Parliament to approve air strikes against IS in Syria.
Mr Corbyn said: "Let us say we will never again unnecessarily put our troops under fire and our country's standing in the world at risk. Let us make it clear that Labour will never make the same mistake again, will never flout the United Nations and international law."
But earlier Mr Corbyn was forced to defend comments in which appeared to compare the actions of IS militants in Iraq with those of the US military during the war
He made the comments in June last year in an interview with Moscow-funded news channel Russia Today.
In a video clip of the interview which surfaced online, Mr Corbyn was asked what could help the Iraqi military regain control of areas seized by IS, also known as Isis.
The Islington North MP replied: "It requires a sense of unity among people in Iraq that want to stay part of Iraq and also an acceptance and an understanding why so many people in so many of the cities in the north have been prepared to accept the Isis forces.
"Yes they are brutal, yes some of what they have done is quite appalling, likewise what the Americans did in Fallujah and other places is appalling."
After his comments attracted sharp criticism, Mr Corbyn was forced to clarify his opposition to the militants.
His spokeswoman said: "Jeremy Corbyn believes the violent ideology of Isis is a vicious, repugnant force that has to be stopped - where Jeremy Corbyn talks about the need for a political solution and compromise he means not with Isis but against Isis, working across the region and beyond to choke off supplies that help fund and arm them and working with neighbouring states in the region to come to common solutions."