The former boss of MI5 has said Iraq was a "distraction" that posed no threat to the UK when Tony Blair took the country to war.
Dame Eliza Manningham-Buller said intelligence suggested the domestic threat could actually increase as a result of the invasion.
The ex-security service chief made the remarks in an interview in which she said she "assumes" there will be bombs attacks in Britain.
"Iraq did not present a threat to the UK," she told the Radio Times about the 2003 war, which was led by the then-American president George W Bush and Mr Blair, who was the British prime minister at the time.
"The service advised that it was likely to increase the domestic threat and that it was a distraction from the pursuit of al Qaeda.
"I understood the need to focus on Afghanistan. Iraq was a distraction," she said.
"Intelligence isn't complete without the full picture and the full picture is all about doubt. Otherwise, you go the way of George Bush," she continued.
Dame Manningham-Buller said it was "for others to decide" whether the war was the right decision.
She has made similar comments to Sir John Chilcot's official inquiry into the decision to go to war .
Referring to the on-going fighting in Libya, she described Colonel Muammar Gaddafi as a "horror" but suggested the UK's warmer relationship with him in the past had some benefits.
"There was a point to cosying up to him, to get him to forfeit his stockpiles of WMD (weapons of mass destruction)," she said.
"It was the right thing to do. But, yes, you do have to be aware of who you're dealing with."
Acknowledging it can be tough to decide when to intervene abroad, Ms Manningham-Buller added: "It's very difficult - do you just stand by and watch people being murdered?"
The ex-spy boss said she "assumes there will be" bombings in Britain in the future.
"This isn't a 'war' you win in a military sense, and you can't anticipate everything," she added.
Her comments coincided with reports a suicide bomber had killed 29 people and wounded 38 others at a Sunni mosque in wester Baghdad.
The bomber blew himself up as worshippers were praying during a special service in the holy Muslim month of Ramadan, which ends on Tuesday.
Officials said Khalid al Fahdawia, a leading member of the Iraqi parliament, was among the dead.
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