'Yes, I'm playing Myra Hindley. It's my duty to raise things we're afraid of'

Actress Samantha Morton had to be talked into taking the role of the Moors Murderer in controversial film

As one of Britain's brightest acting stars, her remarkable array of characters has included a teenage prostitute, a clairvoyant crime-fighter and a mute laundry woman.

But now two-time Oscar nominee Samantha Morton is to brave the most demanding and controversial role of her career by playing the infamous Moors Murderer Myra Hindley in a new film to be screened later this year - the 40th anniversary of Hindley's conviction.

Morton will begin filming the drama, a biopic about the penal reform campaigner Lord Longford, later this month. But the film, commissioned by Channel 4, has re-opened wounds for families whose children were killed by Hindley and her partner in crime, Ian Brady.

Winnie Johnson, the mother of 12-year-old Keith Bennett, who was murdered in 1964 but whose body has never been found, branded the production "disgusting". "When they do things like this it just prolongs the agony," she said.

Hindley and Brady struck five times between 1963 and 1965; their victims, who were tortured before they were killed, ranged in age from 10 to 17 years old. Most of the bodies were hidden on Saddleworth Moor in Greater Manchester.

Both were imprisoned for life. Hindley died behind bars in 2002 at the age of 60, while Brady is still being held in Ashworth mental hospital after being declared insane in 1985.

Morton, 28, who has a five-year-old daughter, admitted she thought long and hard about taking on the role of Hindley. "I initially didn't even want to look at the script. I didn't want anything to do with it," she said. However, she said she was intrigued by Hindley's imprisonment and said that if her role in the killings had been taken by a man, "she would have been out in about 12 years". Morton added: "I believe it is my duty as a performer to raise issues in the world of things we're afraid to look at."

The 90-minute film, called Longford, looks at the life of the former cabinet minister who took up the killer's cause in the face of public hostility. The drama is billed as an exploration of "the reasons behind this eccentric man's steadfast conviction that Hindley should be released and rehabilitated".

Lord Longford began to visit prisoners in his early 30s, motivated by his Catholic beliefs. He campaigned for prison reform and was instrumental in the establishment of the probation system. He will be played by Oscar winner Jim Broadbent, while Brady is to be portrayed by Andy Serkis - who played Gollum in the Lord Of The Rings trilogy and is appearing as King Kong in Peter Jackson's latest film.

Writer Peter Morgan said: "There's a wonderful quote by Bernard Levin, 'Everybody asks the wrong question about Lord Longford, viz, is he barmy? The question is not worth asking. Of course he is barmy. What we should be discussing is, is he right?' As soon as I read that, I knew I wanted to write about him. I want people to ask themselves whether, despite his many faults, isn't it better to live in a world where people like Lord Longford exist and are saying what they are saying and asking these sort of questions. This is the debate I hope this film will raise."

Danny Kilbride, brother of victim John Kilbride, said: "I met [Longford] two or three times and I don't think he made an awful lot of sense. He couldn't really answer any questions about things. No disrespect to his family, but I think he was misguided - he thought he was helping, but he was helping the wrong people."