US music sensation Justin Timberlake has leapt to the defence of ex-girlfriend Britney Spears over pictures of her baby that were published against her wishes on the internet.

Many unofficial sites showed the pictures of Britney, husband Kevin and baby Preston earlier this week, much to Britney's dismay and anguish.

She had changed her mind on having the pictures published, but the sites hijacked the photos anyway, something old love Justin says is unfair.

"When a person like her, who's obviously a sweet person, is having their first child, it's like, 'leave the girl alone, let her have the baby!'" the former *NSYNC star said on US TV show Access Hollywood.

"I do think that's crossing the line. That's her baby and those are her baby pictures."

Britney's representatives were quick to lay down the law regarding the pictures, saying through a statement: "Anyone who publishes, sells or otherwise exploits any of these images in any way will be subject to liability and damages for wilful infringement of copyright, and will be liable for invasion of privacy."

Justin and Britney were both stars of the Mickey Mouse Club on US TV as children and dated when they both became popular pop icons.