The Nanny McPhee star said actresses in their 20s who were looking to reinvent themselves were often told “you’ve got to be thinner”.
In an interview with the Sunday Telegraph, Thompson, 54, said young female screen stars were pressured to be a “specific model size”.
“It seems young actresses are under pressure to look a particular way,” she said.
“They have to be this specific model size, and if they get on to the red carpet, they’re all having to walk like models and dress like models. I think the pressure is terrible.
Thompson, who has a 14-year-old daughter Gaia, also blamed social media for putting “enormous pressure” on children.