Fuller's production company 19 TV today issued writs against Cowell, his production companies Simco and Syco, as well as co-pro firm Fremantle Media, for breach of copyright and contract.
"19 TV will be pressing for a speedy trial to resolve the matters as swiftly as is possible," said a 19 spokesman today.
According to the Media Guardian, 19 is preparing a video compilation to highlight alleged similarities between the two shows - including logo, music, camera angles and direction. It is also claiming that the way contestants exit the audition rooms is identical; the only difference being that instead of Ant and Dec waiting to greet them, it's Kate Thornton - host of ITV2's spinoff Pop Idol show.
The row could threaten the future of both Pop Idol and American Idol, where Cowell is contracted as a judge and Fremantle coproduces with 19.