Kate Winslet, champion of the conceit that you do not have to starve yourself to be a successful actress, has announced she is going on a diet under pain of destroying her career.

Ms Winslet, 25, who recently gave birth to her first child, Mia, has resolved to lose weight or run the risk of not working again in an industry that she said only values thin women.

"It's so insane and bloody boring," she said. "I despise myself for it and feel I'm letting a lot of people down. I constantly wave the flag of 'don't got on diets because they are rubbish'."

She conceded, however, the necessity of losing "a bit of the baby weight or I won't work".

Last year she criticised the pressure put on young women to fit the film industry's skinny ideal. "Girls are brought up to believe that to be thin is to be loved, adored, perfect," the Oscar nominated actress said, in an outburst that won her applause from eating disorder organisations. "That's how they'll get a boyfriend, by becoming like a stick insect. But look at Marilyn Monroe, size 16 and gorgeous. We don't see people like her any more, we just see these perfect thin people."

A report published by the British Medical Association last year on the link between eating disorders and the media, found that actresses had an average of 10% less body fat than healthy women. It concluded: "The gap between the ideal body shape and the reality is wider than ever. There is a need for more realistic body shapes to be shown on television and in fashion magazines."

Ms Winslet said: "I thought I had a way of helping people here, by not getting sucked into that again. I've had lots of letters from mothers who have anorexic daughters, thanking me for the things I've said.

"What annoys me most," she said from her home in Weybridge, Surrey, "is that the more terribly thin and fit we actresses have become, the less real the films become, which is sad. Thank God for the British ones. They don't care what shape you are."