They add that the weight-loss programme, which cuts down on high carbohydrate food, has been "derided by nutritionists and other health-care officials for decades".
By saying that she uses or endorses the diet, the legal letter states, Zeta-Jones is being falsely represented to some people, including "many young women who look up to her and admire her beautiful appearance".
Her attorney, John Lavely, states that association with the Atkins diet may damage her financially. He suggests that it may harm her ability to win endorsement deals for health-related products. Zeta-Jones is already estimated to have product endorsement deals worth £32 million.
Mr Lavely says that the diet has been called "pseudo-science" by some doctors, and has been blamed for an assortment of health problems and other negative side-effects - including constipation, kidney problems, loss of bone density, decreased sex-drive and increased odds of getting certain types of cancer.
He concludes: "In essence, my client is being made to look as if she is more concerned with her outward appearance than she is with serious health concerns. Nothing could be further from the truth."
Earlier this year Zeta-Jones and her husband Michael Douglas brought a High Court action against Hello! magazine. They had wanted £600,000 for commercial damage and distress they suffered over unofficial wedding photographs published by the magazine.
Zeta-Jones complained that one photograph of her dancing "made me look large".
A judge decided that they should get £3,750 each for the distress they suffered, £50 each for a breach of their rights under the Data Protection Act, and £7,000 to cover expenses.
In the past year, 33-year-old Zeta-Jones has completed her transformation from the girl from Swansea into an A-list star.
She won an Oscar for her role in Chicago, attending the Academy Awards ceremony wearing diamonds and heavily pregnant with her second child by Douglas.
The couple are worth more than £150 million, £50 million of which belongs to Zeta-Jones, who has productendorsement deals with T-Mobile phones and Elizabeth Arden cosmetics.