Labour leadership contender Ed Balls became the first former cabinet minister to unequivocally come out and say the Iraq War was "wrong".
In an interview with The Daily Telegraph, the former children's secretary said the decision to join the invasion in 2003 was an "error" for which Britain had paid a heavy price.
"It was a mistake. On the information we had, we shouldn't have prosecuted the war. We shouldn't have changed our argument from international law to regime change in a non-transparent way," he said.
"It was an error for which we as a country paid a heavy price, and for which many people paid with their lives. Saddam Hussein was a horrible man, and I am pleased he is no longer running Iraq. But the war was wrong."
Mr Balls, who was an adviser to Gordon Brown in the Treasury at the time of the invasion, said he had warned his boss it was a mistake to try to blame the French for the break down of international negotiations to find a peaceful resolution.
"I was in the room when a decision was taken that we would say it was that dastardly Frenchman, Jacques Chirac, who had scuppered it," he said.
"It wasn't really true, you know. I said to Gordon: 'I know why you're doing this, but you'll regret it.' France is a very important relationship for us."
His comments will be seen as an attempt to distance himself from one of the most controversial legacies of the former Labour government.
Despite his reputation as Mr Brown's closest lieutenant, he insisted they had often argued. He said: "I had more blazing rows with him than anyone. You had to do that sometimes to shut Gordon up."
Mr Balls also confirmed that he had offered to stand aside to allow his wife, former work and pensions secretary Yvette Cooper, to run for the leadership, but she had declined.