In extracts from his memoirs titled 'A Journey' Blair says he's wept over the loss of life adding that he didn't anticipate the role of Al-Qaeda or Iran in post-invasion planning.
In his account of his decade in office he also says he still believes that leaving SADDAM in power would have been a bigger risk to security than removing him.
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He'll donate all the proceeds of the autobiography which hits shops today to a charity for injured war veterans.
The news comes as US Defence Secretary Robert Gates has arrived in Iraq to meet with US troops as they formally close down their combat mission.
The Pentagon chief's plane landed at Al-Asad Air Base, about 160km west of Baghdad, just a few hours after President Barack Obama told Americans it was time to "turn the page" on Iraq.
Fewer than 50,000 US troops remain, down from more than 165,000 at the height of the fighting. The remaining forces' primary role is to help train and equip Iraqi forces over the next year.
Gates flew to Iraq from Milwaukee, where he told an American Legion audience on Tuesday that Iraq's problems had not all been solved.
Still, he expressed "pride on what our troops and their Iraqi partners have accomplished.
