Roberts is counter-suing her accusers

They have filed a lawsuit claiming they were paid half of what they were promised for playing on the set of Mona Lisa Smile, reports the New York Post.

Roberts and her production company have reportedly counter-sued accusing them of trying to "embarrass" Ms Roberts.

A spokesman at Manhattan Supreme Court confirmed papers had been filed.

Julia Roberts plays a professor who gets women to stand up for themselves, and here we are being discriminated against Jeanne Daly, plaintiff

The four women - jazz musicians named as Jeanne Daly, Jill McCarron, Ada Rovatti and Lauren Sevian - have also reportedly filed complaints with the Equal Employment Opportunity Commission and the Screen Actors Guild.

Ms Daly told the newspaper they had been initially offered £440 for each eight-hour shift, but this was later changed to £220 for 12-hour shifts.

The women say they discovered that male jazz players in the film were each getting £350 a day.

Roberts' production company Red Om Films has counter-sued, accusing the women of trying to "harass, embarrass and annoy defendants in order to extract a sizeable settlement".

In the film Roberts plays an art history professor at a women-only college who persuades her students to seek more from life than the perfect husband.

Ms Daly is quoted as saying: "Julia Roberts plays a professor who gets women to stand up for themselves and break out of their stereotypical roles. And here we are being discriminated against. It's pretty pathetic."