CBS News discovered the post in the "al Hesbah" chat room. The post, almost 300 words long, was left by a regular contributor who uses the name abu Osama al-Hazeen, the U.S. broadcaster said.
"We say to Britain: The Emir of al Qaeda, Sheikh Osama, has once threatened you, and he carried out his threats. Today I say: Rejoice, by Allah, London shall be bombed," the message reads.
The message attacked Britain for granting a knighthood to the Pakistani-born writer Salman Rushdie.
Peter Clarke, head of Scotland Yard's counter-terrorism unit, said that no intelligence warnings of an impending attack had been picked up before the first car bomb was discovered by an ambulance crew. A second car was ticketed for illegal parking and towed to an underground garage, where employees noticed the smell of gasoline and discovered the bomb materials.