Kelly Clarkson's fellow American Idol finalist Justin Guarini claims they had a secret relationship.

The "Because of You" hitmaker - who is expecting her first child with husband Brandon Blackstock - won the first series of the show in 2002, and the runner-up claims they began a romance when they starred together in movie From Justin to Kelly, but separated when her career grew more successful while his stalled.

According to a review posted on website Mcall.com, Justin told the audience at his Lovesick one-man cabaret show in Pennsylvania last week about the romance, and also with another Idol contestant from his series, the fourth-placed Tamara Gray. Justin - who has two children with wife Reina - has previously described Kelly as being "like a sister" to him. "We've just always been such good friends. Um, man, it was weird; she was like my sister on the show, and then you know, we made a movie together. ... I'd say, we had a good time. Yeah," he previously said in 2012.

And speaking around the release of From Justin to Kelly, the "Stronger" singer was quick to dispel talk of a romance with her co-star. "We're like brother and sister," Kelly insisted at the time.