Discworld author Terry Pratchett has spoken out against new guidelines on assisted suicide which were issued last week.

The guidelines do not provide immunity from prosecution – assisting suicide carries a prison sentence of up to 14 years – but are intended to offer clear advice to the relatives of people wishing to kill themselves about whether they would face prosecution.

Pratchett, who revealed in 2007 he was suffering from early-onset Alzheimer's, said yesterday the new guidelines had him "a little more angry". "No one is really happy with them," he told the BBC Politics Show West. "It would appear they are suggesting that people could help you kill yourself, and then the police would investigate as an act of murder and decide whether or not this was really the case, which seems a very lame way of doing things."

The bestselling fantasy author suggested instead "that there should be possibly some kind of non-aggressive tribunal system where someone who, for whatever reasons, wishes to end their life – and I would only really accept medical reasons, I must say – can make their points to a magistrate or a coroner, along with the medical evidence on which they wish to end their life". This, Pratchett felt, would "protect the vulnerable", and "could weed out the hypothetical granny who is being urged by her heirs to commit suicide, so they can get their hands on her money".

Pratchett said that he himself was "feeling fine", and that although he can no longer drive a car or use a typewriter, he is continuing to write using a speech-to-text programme. "I can now talk to the typewriter and get the work done," he said. Pratchett's new Discworld novel, Unseen Academicals, is published in October.

"The reason I wish to end my life is because of a medical condition – it's not bad yet and I'm full of the joys of spring, but it will eventually get very bad and Alzheimer's is the most feared disease among the over-60s. It is not nice. I do not wish to be there for the endgame," Pratchett said. "I spoke to a doctor who told me when he was a young doctor he walked through a ward of terminal Alzheimer's patents who effectively were not there any more, and he wondered why they were being kept alive, when there was very little there to keep alive."

A 12-week consultation on the new guidelines is currently ongoing, with the finalised policy to be announced next spring.