The mother of Keira Knightley, the actress, has hit out at a whispering campaign suggesting her daughter has an eating disorder.

Sharman Macdonald, the Scottish playwright, blamed “playgound bullies” for spreading persistent rumours about her daughter’s waif-like physique.

Macdonald said her 23-year-old daughter, best known for her roles in the Pirates of the Caribbean series, Bend It Like Beckham and Atonement, has a genetic predisposition to thin-ness, inherited from her father.

She says Knightley has been forced to adopt a diet high in protein and carbohydrates to ensure that she does not become too thin. Like her father Will Knightley, a theatre actor, she must “eat like a horse” to maintain a healthy body weight.

“She has always been thin. She’s her daddy’s daughter, with his long body,” she said. “Daddy was much, much thinner than Keira. When he was Keira’s age, he had to drink milk with honey and eggs, and go training and training and training, just to be a normal weight.

“She eats like a horse. I always want to apologise because she can eat anything that she wants and she does not put on weight.”

Macdonald said Knightley was put on a special diet to help her bulk up for her role as Guinevere in the 2004 film King Arthur, but had been forced to abandon it because she was eating less than she normally did and was losing weight rather than putting it on.

“When she was doing King Arthur, when she was still living at home, they gave me a diet sheet to help her put on weight,” said Macdonald.

“I looked at it and thought: ‘If she follows that, she’s going to lose weight. It’s less than she eats normally, but they must know what they’re doing.’ So we followed it. The weight was dropping off her. I had to say: ‘Right, just eat what you did before, and start drinking protein shakes as well.”

Despite her attempts to increase her size, Knightley remained stick-thin and her bust was digitally enhanced for American posters advertising the film.

The Oscar-nominated actress has repeatedly denied claims she suffers from an eating disorder, insisting she is “naturally thin”. While Knightley has admitted that both her grandmother and great-grandmother suffered from anorexia, she said she had not inherited the condition.

Commenting on her size recently, Knightley said: “Anybody with an ounce of intelligence would say that too much emphasis is put on weight . . . I’ve been on the screen since the age of seven and you can see what my body type is if you want to. I am not anorexic. I have never been anorexic and I do not have any eating disorder.”

Last year, Knightley successfully sued a tabloid newspaper over a “deeply offensive and embarrassing” article that implied she had an eating disorder and was responsible for the death of a 19-year-old anorexic because she had set a bad example

Macdonald said she found the false accusations distressing, but felt powerless to stop them.

“I can’t bear it at all and neither could you. When kids are bullied in the playground, their parents are in absolute agony,” she said. “It’s a playground situation that we’re looking at with the press, it’s a form of bullying. But what can we do? She has sued once, and she won. If she did it again, she would win again. But you can’t keep doing that. It would take over your life. So you have to just turn away.”

Last week, dieticians said that some people were genetically predisposed to be thin.

Lorraine McCreary, a registered NHS dietician who runs Diet Scotland, a clinic specialising in weight loss and eating disorders, said: “New genes that code for obesity are being discovered all the time, and in the same way there will be genes that code to keep people very lean.

“You do get very slim people who can eat an awful lot of calories and maintain a very lean body mass. If Keira is not avoiding eating, and her eating pattern is normal and her father had that as well, then it’s very likely that there is a genetic link.”

Knightley’s next role is as the lover of Welsh poet Dylan Thomas in The Edge of Love. The film will receive its premiere at the Edinburgh International Film Festival on Wednesday.