Princess Margaret's naughty side revealed in Channel 4 comedy

Princess Margaret was the first modern Royal to scandalise the nation, provide endless fodder for the gossip columns and yet still earn considerable public sympathy over her search for contentment.

Now The Queen's Sister, a two-hour comedy/drama for Channel 4, to be shown on 27 November, tries to capture the contradictions of a woman torn between public and private life and forever destined to live in the shadow of The Queen. "Disobedience is my joy; I've always been the naughty one," she says. "I did mind forever being cast as the younger sister." The film stars a relative unknown, Lucy Cohu, in the main role, for which she is likely to be widely praised, and with two other respected actors, Toby Stephens as her husband, photographer Anthony Armstrong Jones, now Lord Snowdon, and Shameless star David Threlfall, who plays the Duke of Edinburgh.

The Queen's Sister is a new approach by dramatists and film makers to mine the potentially fascinating subject of the post-war Windsors. It will be followed by Stephen Frears's film The Queen, starring Helen Mirren, which will focus on events after the death of Diana, Princess of Wales in 1997. Although Princess Margaret died in 2002, many of the key characters are still alive, including Lord Snowdon, now 75, and one-time lover Roddy Llewellyn, a garden designer, played by newcomer Simon Woods as a gauche semi-hippie.

Many believe the traumas of Princess Margaret's life dated from her decision not to marry Peter Townsend, an RAF officer who was her first love, but also a divorcée. The film, written by Craig Warner, suggests she took the decision not just because of official disapproval but because a civil marriage would mean her losing all the trappings and financial support of being a Royal. She consoled herself among the nightclubs and young bucks of a London emerging from post-war gloom, causing scandals and starting a fashion when she smoked in public. Much of the film is devoted to the tempestuous relationship between the Princess and Lord Snowdon, forced to reconcile his desires for a normal life with the demands of Royal duties. This means, for instance, addressing his wife as Ma'am when the butler is around, having to wear a jacket for dinner and always walking behind her.

The film, unthinkable in its nudity and frankness about Royal misbehaviour even a couple of decades ago, charts the meeting of Armstrong-Jones and the Princess at a high-society swingers evening, through their joint affairs and eventual divorce. Channel 4 confirmed yesterday that Lord Snowdon had been offered an advance screening, but rejected suggestions that it had negotiations with his lawyers.

Princess Margaret's persistent misbehaviour reaches a point where she is told by the Duke of Edinburgh that her behaviour has become an embarrassment and she must make herself scarce, freeloading at the homes of friends, enlivened by the affair with Llewellyn, and the endless holidays on Mustique.

The film also shows a younger audience, who may have believed Royal scandals began and ended with Diana, that unlikely marriages, infidelity and betrayal have always been part of the Royal picture.