On the eve of her film's planned premiere at the Cannes Film Festival,

Nicole Kidman isn't getting any love for her  Grace Kelly  biopic.

The Philadelphia girl turned Hollywood golden girl turned real-life princess -- thanks to her marriage to Prince Ranier in 1956 -- was killed in a car crash in 1982.

Her story, with Kidman in the title role, would seem to be a natural for the big screen, but now Grace's royal heirs are slamming the film, and the director and  distributor are battling over the final cut, with the possibility Grace of Monaco will be dropped from the prestigious Cannes lineup.