Keira stars in her mother's film

Sharman MacDonald with her daughter Keira Knightley

Hollywood actress Keira Knightley's latest film is a family affair - it was written by her mother.

The Edge of Love, by Glasgow-born playwright Sharman MacDonald, will open the Edinburgh International Film Festival on Wednesday night.

The film stars Sienna Miller as Caitlin, the wife of Welsh poet Dylan Thomas, and Knightley as Vera Killick, his childhood sweetheart.

MacDonald said her daughter was determined to get the film made.

The story is based on a single incident related to MacDonald about William Killick, Vera's husband, shooting a gun through the window of the Thomas's home.

Knightley, 23, plays Vera but when her mother started writing the screenplay she had her daughter in mind as Caitlin.

MacDonald, who lived in Glasgow and Ayrshire during her childhood and went to Edinburgh University, said: "I started writing it when Keira was 17. I was writing it on St Vincent where they were shooting Pirates of the Caribbean.

"She is a very spirited, humorous, live-loving girl so all that fed into the creation of Caitlin. But she was only 17, so if the film had been made right then she could not have played any of the parts.

"As time went on she became my idea of Caitlin. I thought 'if this is made perhaps she will play it'."

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Keira Knightley on the Edge of Love

MacDonald said that Knightley read the script and was desperate to get it made.

"It is absolutely wonderful when your children love what you do," she said.

Director John Maybury was wooed with cake and champagne by Knightley and when a studio got involved Knightley chose to play Vera and not Caitlin.

Macdonald said she did not watch her daughter shooting the sex scenes in the film.

She said: "I have never gone near any of the films when she has either been kissing anybody or shooting a sex scene.

"I have seen them afterwards but then she is the character, not my daughter, so it is fine."

MacDonald said she is very proud of her daughter and dismissed speculation about her being anorexic as "hurtful nonsense".