Eddie Izzard reveals why he is so driven

Eddie Izzard, the comedian-turned-actor who has run 43 marathons in 51 days and now harbours political ambitions, has revealed the reason why he drives himself so hard: the death of his mother, when he was aged just five years old.

In a documentary, Believe: the Eddie Izzard Story, to be shown in cinemas next month, Izzard breaks down on camera when he is asked why he continues to push himself despite his success as a two-time Emmy-winning comedian.

In an emotional on-camera confession to the director Sarah Townsend - who is his ex-girlfriend - Izzard reveals: "I keep thinking that if I do all these things, and keep going and going, then... she'll come back" before he bursts into tears.

Izzard, now 47, suffered 15 years of rejection before he became a successful stand-up. He learned French in order to get gigs in France, and then relocated to America to pursue a film career that includes Ocean's Twelve, Ocean's Thirteen and The Chronicles of Narnia: Prince Caspian.

Last year he put himself forward for the role of the 11th Doctor Who, which eventually went to Matt Smith. A relentless campaigner, Izzard did a seven-week marathon throughout the UK in July to raise money for Sport Relief. At the end of his 43 marathons, at a reception in Downing Street, he announced that his future lies in politics, and that he plans to stand - presumably for Labour, to whom he is a major donor - in either two or three elections' time.

Izzard has by no means shied away from talking about his mother Dorothy Ella Izzard, a midwife and nurse, in his 15-year career. Izzard, who was Dorothy's youngest child, mentions her in his stand-up routine and in interviews, and also named his production company, Ella, after her. But this is the first time he has broken down publicly about her death.

Talking about the Believe documentary, he told the Times: "I didn't know I was going to say that, because... I didn't know I thought it. That's why it's weird. That's why I start crying."

Believe: the Eddie Izzard Story will open at some cinemas in Britain on December 11. View the trailer here.