Britain and the United States have agreed to launch a joint programme to develop drugs to treat the condition.

During a visit to New York today, the Prime Minister will say that hundreds of thousands of elderly people live with conditions such as Alzheimer’s without receiving help to alleviate its symptoms.

Government sources said the PM was pushing to increase diagnosis rates to 67 per cent by 2015. The current rate is 45 per cent. There are estimated to be 670,000 people in England suffering dementia, of whom 350,000 remain undiagnosed, without support.

Mr Cameron will warn that the condition is fast becoming the biggest source of pressure on care systems around the world. There are thought to be 35.6 million people worldwide living with dementia, but the number is projected to soar to 65.7 million in 2030 and 115.4 million in 2050. A G8 dementia summit will be held in London in September, bringing together health and science ministers with medical and pharmaceutical experts.

Mr Cameron said last night: “Dementia is a devastating disease – not just for sufferers but for their families and friends too. And as more people live longer, it is fast becoming one of the biggest social and healthcare challenges we face. Families, communities, health systems and their budgets will increasingly be strained as the number affected increases and so we need to do all we can to improve how we research, diagnose and treat the disease.”

The Health Secretary, Jeremy Hunt, added: “For too long diagnosis rates have been shockingly low, leaving too many people living in the dark trying to cope with this terrible condition undiagnosed, unable to get the help they need.

“Dementia is a serious and growing problem so this ambitious drive to see a clear majority of people identified and supported is a major step forward.”