Testifying on Wednesday before the bipartisan commission probing the 2001 attacks that killed 3000 people, Clarke said the Bush administration which took office in January 2001 did not consider the issue urgent.
"I believe the Bush administration in the first eight months considered terrorism an important issue, but not an urgent issue," Clarke, who served the last four US presidents, said.
Clarke's testimonial came only days after his book, in which he directly criticises Bush for part of the September 11 failure, hit the stands making waves.
"I believe the Bush administration in the
first eight months considered terrorism
an important issue, but not an urgent issue"
former US counter-terrorism chief to Bush
Clarke told the national commission Bush had "greatly undermined the war on terrorism" by invading Iraq last year.
Seeking to discredit Clarke, the White House released the transcript of a briefing Clarke gave in early August 2002 praising the way the Bush team had taken over the war against al-Qaida.
A fuming National Security Adviser Condoleezza Rice also accused Clarke of shifting position from praising Bush to criticising him. Some Republicans on the commission also said Clarke had damaged his credibility by sensationalising his charges against Bush for personal gain.
The commission of five Republicans and five Democrats earlier heard CIA Director George Tenet.
Asked repeatedly why the agency failed to thwart the attacks, Tenet said: "We didn't integrate all the data we had properly, and probably we had a lot of data that we didn't know about that if everybody had known about maybe we would have had a chance. I can't predict to you one way or another."