British officials were told by the FBI and New York police before the July 7 attacks of a warning that suicide bomber Mohammed Sidique Khan "was trouble" and should be "checked out", it was reported yesterday.
The New York Daily News quoted sources saying US agencies were given the tip-off by a convicted terrorist-turned-informant.
According to the newspaper, British investigators initially followed Khan, but may have disregarded him because they thought he was not a pivotal player in a radical group.
The informant, who cannot be named for legal reasons, pleaded guilty in the US in June 2004 to providing material support to al Qaida.
NYPD spokesman Paul Browne declined to comment on the latest story.
Khan, 30, was one of four bombers who killed 52 innocent people in the attacks in the summer.
The Home Office and Scotland Yard both refused to comment.