The number of people with dementia and Alzheimer's disease is soaring around the world and will nearly double in the next 20 years, according to a report published today.

The report, by a team of researchers led by Professor Martin Prince of the Institute of Psychiatry at King's College London, says that by next year there will be 35 million people globally with dementia. By 2030, that will have risen to 65.7 million and the steep rise will continue to more than 115 million by 2050.

The burden on those with the disease, on their carers and on society is immense, according to the report, which is published by Alzheimer's Disease International (ADI). It calls on the World Health Organisation to make dementia a world health priority in the hope that this will spur countries to produce national plans for coping with the disease and encourage research into treatments.

The true scale of dementia around the world is hard to establish because many low- and middle-income countries do not keep good statistics. The figure of 35 million is a revised estimate, and 10% higher than that stated in a review in the Lancet medical journal in 2005.

The proportion of older people affected has risen in three regions of the world – in south Asia (up to 5.7% from 3.4%), Latin America (up to 8.5% from 7.3%) and western Europe (up to 7.3% from 5.9%). The biggest increase in future years is expected to be in low- and middle-income countries, as people start to live longer.

Prince emphasised the enormous indirect toll dementia can take on carers, who are often unpaid family members. "Caring is a full-time job – an average of around eight hours per day for a relative with moderate to severe dementia," he said.

"In all parts of the world, carers – who are most commonly female and the spouses or children of the persons with dementia – often experience high levels of strain.

"Studies reviewed in the new report suggest that half to three-quarters of carers have significant psychological illness, while up to a third have clinical depression."

He said that given these statistics, "current investment in research, treatment and care is actually quite disproportionate to the overall impact of the disease".

Marc Wortmann, ADI's executive director, said much more could be done by governments to alleviate the burden of the disease on patients and their families.

"The crisis of dementia and Alzheimer's can no longer be ignored. Unchecked Alzheimer's will impose enormous burdens on individuals, families, healthcare infrastructures and the global economy," he said.

"There is hope yet, if action is taken now to fund improvements in dementia care services, and to increase investment in research."