He said that immediately prior to the U.S.-led invasion, his weapons inspection team checked some 40 Iraqi sites that, according to U.S. and British intelligence, housed weapons of mass destruction.
But his team never found any such evidence.
"They should have realized, I think, both in London and in Washington that their sources were poor," said Blix.
"Their sources were looking for weapons, not necessarily weapons of mass destruction. They should have been more critical of that," said Blix.
Of Blair's decision, Blix said, "I certainly felt that he was absolutely sincere in his belief. What I questioned was the good judgment."
The British inquiry into Iraq was set up a year ago to examine the run-up to the Iraq war, the military action and its aftermath. It aims to identify what lessons can be learned for the future.