The French company was in court last week defending themselves against a charge of racism regarding only using white women to promote their Garnier shampoo products.
Following the guilty verdict, Winslet and actor Clive Owen, who are the two new faces of the brand, are being urged to break their contracts with L’Oreal as a stand against racism.
A spokesperson for race rights said, "We would ask those high-profile ambassadors to think carefully about whether or not they want to be associated with this organisation. What a wonderful opportunity this could be to stand up to racism, to show that it simply has no place in today’s society."