'Duets' judge John Legend trashes Britney Spears and Demi Lovato

There have been some pretty noted music rivalries through the years.  Biggie versus Tupac. Eminem versus Moby. Courtney Love versus Dave Grohl. But there's a new one emerging on the horizon - John Legend versus Britney Spears and Demi Lovato.

The judge on the upcoming singing competition "Duets" fired the opening salvo while at a recent red carpet event. It is here that he tells E! that he questions the credibility of the newly-minted "X Factor" panelists.

"It's interesting when people, you know, are judging singing competitions when they aren't really singers," Legend says.

Ouch.

Spears recently signed a contract thought to be in the $15 million range to judge contestants on "X Factor." Lovato, who was a late entrant into the judging derby, inked what is believed to be a deal for about $1 million.

As for Legend, the soulful singer will be joined by Robin Thicke, Kelly Clarkson and Jennifer Nettles on the ABC program. That series kicks off on May 24 at 8 p.m. ET.