Former Foreign Secretary Jack Straw has described the current situation in Iraq as "dire".
Mr Straw, who held the job at the time the UK decided to take part in the 2003 war, said there were things he regretted about the campaign.
Speaking on BBC One's Question Time, he said "mistakes" were made by the US following the invasion.
State department efforts to ensure a "proper civilian administration" were not followed through, he said.
"The current situation is dire," he said.
"I think many mistakes were made after the military action - there is no question about it - by the United States administration. Why? Because they failed to follow the lead of Secretary (of State, Colin) Powell.
"The State Department had put in a huge amount of effort to ensure there was a proper civilian administration put in straight away afterwards."
Mr Straw, now Leader of the House of Commons, said some people would see the Iraq war as "Tony's folly" but that was not a view he believed would stand "in time" about Prime Minister Tony Blair's role.
Margaret Beckett replaced Mr Straw as Foreign Secretary in the May 2006 reshuffle.
Mr Straw said that while he felt the current situation in Iraq was "not satisfactory" he had expressed such a view before leaving the post.
"I certainly said there were mistakes made," he told Question Time.
Mr Straw added that there were people in the US administration in 2003 who wanted to invade Iraq "in any event" but he did not believe President George W Bush was one of them.
"The thing that people forget in this situation is the successful efforts Tony Blair made, which I played a part, to shift the American administration from that position to one where we took it to the United Nations," he said.