Sir Terry Pratchett, the fantasy writer who was diagnosed with Alzheimer's in 2008, said yesterday he had started the formal process that could lead to his own assisted suicide at the Dignitas clinic in Switzerland.

Pratchett, whose BBC2 film about the subject of assisted suicide is to be shown on BBC2 tomorrow, revealed he had been sent the consent forms requesting a suicide by the clinic and planned to sign them imminently.

"The only thing stopping me [signing them] is that I have made this film and I have a bloody book to finish," he said during a question-and-answer session following a screening at the Sheffield documentary festival Doc/Fest.

He said that he decided to start the process after making the film Terry Pratchett: Choosing to Die, which shows the moment of death of a motor neurone sufferer, millionaire hotel owner Peter Smedley.

Pratchett, the creator of the Discworld novels who was 60 when he was diagnosed, said his decision to start the formal process did not necessarily mean he was going to take his own life.

According to Dignitas, 70% of people who sign the forms do not go through with taking their own lives.