Longer lifespans means in 20 years there will be nearly twice as many people suffering from dementia in the world as there are today, experts have warned.

In 2010 there will be 35.6 million people across the world with dementia and Alzheimer’s disease, which will almost double to 65.7 million in 2030, according to British researchers. They estimated that the number would nearly double again 20 years later, to 115.4 million in 2050.

Currently about 700,000 people in the UK have dementia, more than half having Alzheimer’s. Earlier predictions said that number would touch a million in less than 20 years, and hit 1.7 million by 2051.

The work was published by Alzheimer’s Disease International (ADI) in the World Alzheimer’s Report 2009.

Leader of the research, Professor Martin Prince from the Institute of Psychiatry at King’s College London, said the findings highlighted pressure on dementia carers.

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.’