In an interview with the BBC radio on Sunday, David Kay said the need for more troops was due to the false assumptions upon which Iraq's invasion and occupation had been planned.
The former chief arms inspector also said the situation in the Shia-majority city of Najaf was "beginning to look like a genuine civil rebellion'' with fighters "numbering in thousands".
Kay said the US needs to send in more troops to keep the peace, many more that its present deployment of 135,000 troops in Iraq.
"We need at least 200,000, probably 250,000 on the ground if you're going to provide security,'' Kay said on BBC Radio 4's The World This Weekend.
The Iraqi National Congress "told us what we wanted to hear in order to get the course of action that they wanted. I don't blame them, I blame us. We allowed ourselves to be fooled''
Former Iraq Survey Group chief
The much-publicised handing over of limited sovereignty to Iraqis at the end of June is unlikely to improve the situation, according to the ex-official.
"It's hopeless to believe you can turn this over on 30 June if the level of security remains what it's been the last three or four weeks."
Selective info gathering
Kay added the US had clearly acted on "false assumptions about how the Iraqis would react once we entered the country".
He suggested over-reliance on Iraqi refugees and particularly the Iraqi National Congress for providing misinformation.
"They told us what we wanted to hear in order to get the course of action that they wanted. I don't blame them, I blame us. We allowed ourselves to be fooled.''