LONDON (Reuters) - Top-selling author Terry Pratchett will deliver a petition to British Prime Minister Gordon Brown on Wednesday calling for an urgent increase in funding for dementia research.

Pratchett, 60, creator of the "Discworld" series, revealed last December he had been diagnosed with a rare form of early-onset Alzheimer's disease.

"I am appalled that research into Alzheimer's and related diseases, which affect 700,000 people in the UK, currently receives just 3 percent of government medical research funding," he said in the petition.

"I strongly urge the government to increase funding for dementia research as a matter of urgency."

Around 20,000 people have signed the petition which has the backing of over 100 leading scientists, the Alzheimer's Research Trust said.

Pratchett, who is Patron of the trust, said the world faced a "tsunami" of Alzheimer's and other dementia diseases.

"I intend to scream and harangue while there is time," he said.

"The strain on carers and their support is bad enough now -- before very long the effects on the health service and society itself, will be unbearable."

The trust said dementia costs the British economy 17 billion pounds (S$39.56 billion) a year.

"If scientists could work out how to delay the condition by five years, we could halve the number of people who die with dementia," said its chief executive Rebecca Wood.

The government says it will continue to invest in Alzheimer's research and that it has spent 29 million pounds studying dementia and related neurological conditions in the 2006-2007 financial year.

Last month, the publicly funded Medical Research Council and the independent Wellcome Trust announced 30 million pounds of funding for neurodegenerative diseases such as Alzheimer's, multiple sclerosis and Parkinson's disease.