Actress Salma Hayek is set to pile on the pounds for her new movie role - a 140-kg, sex-mad psychopath.

The screen beauty has agreed to make herself fat and ugly for her depiction of real-life serial killer Martha Beck in Hollywood thriller "Lonely Hearts".

Beck and her accomplice Raymond Martinez Fernandez, branded the 'Lonely Hearts Killers', viciously murdered 17 women in the 1940s.

The pair attracted their prey - lovelorn war widows - by placing personal ads in local newspapers in which Ray would describe himself as a sexy Latin lover.

A Hollywood insider revealed: "Beck was a big woman with a big sexual appetite. Salma won't put on all the weight - she will also have padding."