Playwright Sharman Macdonald said the star of the "Pirates of the Caribbean" trilogy has always had to eat a lot of food to gain weight and her genetics likely produce the 23-year-old actress's waif-like appearance, The Sunday Times of London reported.

"She has always been thin. Macdonald said. "She's her daddy's daughter, with his long body."

Macdonald said her daughter did try a strict diet while preparing for her role in the film "King Arthur," but was forced to abandon it when her weight became dangerously low.

The concerned mother also lashed out at critics and the media who have accused Knightley of having an eating disorder such as anorexia.

"It's a playground situation that we're looking at with the press -- it's a form of bullying -- but what can we do?" Macdonald told the Times.