Nicki Minaj threatened Mariah Carey because her 'American Idol' co-star reportedly called her a bitch.

Show bosses cancelled filming in Charlotte, North Carolina for the day after a huge blow up between the pair where Nicki threatened and swore at Mariah but witnesses insist the 'Hero' hitmaker started it.

A source told ''Nicki Minaj walked off of set because Mariah Carey kept making jabs at her. Nicki's manager and team convinced her to come back to the show. Mariah kept at it and called Nicki a bitch and then proceeded to cuss her out. Producers had to stop filming and call it a night.''

It was previously revealed Nicki fumed she was sick of her co-star's ''f*****g highness'' and their fellow panelists Keith Urban and Randy Jackson were caught in the middle.

A source told TMZ: ''[Nicki said to Mariah] 'I'm gonna knock you out'.

''Nicki added 'I told them, I'm not f*****g putting up with her f*****g highness over there.''

Producers are now worried about how the pair will manage to work together for the rest of the season.