Britney Spears Dancer -- You Broke My Nose And DIDN'T CARE

A dancer claims a disheveled and confused Britney Spears backhanded her in the face during a rehearsal, and Brit was so heartless ... she didn't even stop the rehearsal.

Dancer Dawn Noel will file a lawsuit Monday ... claiming she was hired to shoot the music video for "Work Bitch." Dawn says on August 19, she showed up at the rehearsal studio in Woodland Hills, CA. She and the other dancers perfected their moves for several hours, and then Britney arrived.

Dawn says Brit showed up "in a disheveled and confused state," and had considerable difficulty executing even basic moves.

Dawn claims she was standing off to Britney's side, when the singer twirled while unbalanced, extending her hands and SMACKING Dawn in the face.

Dawn says the impact was so forceful, everyone in the room heard a cracking sound. Dawn says she was in excruciating pain, but all Britney said was, "I'm sorry," and then continued the rehearsal.

Dawn claims after a half hour she couldn't take the pain and went to a doctor, who told her she had a nasal bone fracture and needed surgery.

Dawn claims Britney's people promised to pay the med bills, but after months of waiting she was told to pound sand. So Dawn's lawyer Sark Ohanian from Carpenter, Zuckerman & Rowley will file a lawsuit Monday against Britney.

We called Britney's people ... so far, no comment.