The show has been a huge hit despite Rik's exit
Rik Waller, who pulled out of the competition through illness, risks being taken to court by releasing his first single next month.
Thames Television claims he is contractually obliged to wait three months after the end of the series before releasing a record.
But Waller has sent a defiant message to Pop Idol bosses - who include show judge Simon Cowell - saying he never signed the final contract.
The 20-year-old from Kent, who weighs 20st, enjoyed huge popular support but was forced to leave the ITV show with a throat infection.
He has since signed a £400,000 record deal with industry giants EMI and is planning to release a version of I Will Always Love You - a hit for Whitney Houston - on 4 March.
Waller says: "They've sent me a threatening letter saying that if I continue to disobey the terms of the contract they will take legal action.
Number One
"But I didn't sign the final 10 contract, which was the only one legally binding.
"The way I see it I'm not under any obligation whatsoever.
"When I received the letter I thought: 'Go ahead, make my day.'"
He wants his single to knock the Pop Idol winner's first record, released one week before, off top spot.
Steve Pritchard, director of EMI records, said: "We've had no contact with Thames TV but we are expecting a letter from them.
"Rik's single will be out on 4 March and will be number one on Mothers' Day on 10 March."
A spokeswoman for Thames Television said it was too early to say whether legal action would be taken.
"The thing about Pop Idol is that lots of new stars have been pulled from the show and we are very happy about that," she added.