Blix told The Guardian newspaper that he was not accusing Blair of acting in bad faith, but said that the prime minister relied heavily on intelligence reports about Saddam Hussein's weapons of mass destruction.
"What I am saying is that there was a lack of critical thinking," Blix told the newspaper from his home in Stockholm (search), Sweden.
Blair, the closest ally of President Bush in the Iraq conflict, cited Saddam's pursuit of banned weapons as the main justification for taking Britain to war. No such weapons have been found so far.
Blix said U.N. inspectors ought to have been allowed to continue their work for suspected weapons programs, which could have led to a more accurate analysis of the intelligence being received by Britain and the United States.
"Gradually (the British and U.S. governments) ought to have realized there was nothing," he said. "Gradually they would have found that the defectors' information was not reliable."
Blair, whose popularity has slumped since the conflict, defended his position again Friday when he warned that governments could not "err on the side of caution" when dealing with the threat of global terrorism.