The number of cases of dementia world-wide is likely to double every 20 years, say researchers.
An ageing population in China could see many cases
A report for Alzheimer's Disease International, published in The Lancet, found a new case of dementia arising every seven seconds.
It estimated 24.3 million people currently have dementia, with 4.6 million new cases being diagnosed each year as the population ages.
By 2040 the number is predicted to have risen to 81.1 million.
The report said most people living with dementia are in the developing world, with five million in China alone.
This compares with 4.8 million in Western Europe and 3.4 million in North America.
The study also highlighted how the rate of growth of dementia in the developing world already far exceeds that of richer countries.
60% of people with dementia live in developing countries
This figure will rise to 71% by 2040
The numbers in developed countries are set to increase by 100% between 2001 and 2040
In India, China, and their south Asian and western Pacific neighbours rates will rise by more than 300% during this period.
The increase is predicted to be three to four times higher in developing regions than in developed areas.
By 2040 there will be as many people with dementia in China alone as in the whole of the developed world put together.
Orien Reid, ADI chairman, said: "We are faced with a ticking time bomb.
"Governments must start to plan policy, and allocate health and welfare resources for the future.
"Another cause for concern is that millions of these cases of dementia are going undiagnosed - meaning that, through ignorance and stigma, people aren't receiving the care and services that they so desperately need."
Professor Martin Prince, who co-ordinated the study, said: "There is already a great need for community-based services, welfare and support, and these new figures show that pressure on governments for dementia services will increase dramatically in the next few years.
"This need has to be addressed now."
The report concluded there is a fundamental lack of awareness among policymakers, clinicians and the public.
Action needed now
Neil Hunt, chief executive of the Alzheimer's Society, said: "Dementia is becoming one of the world's biggest health problems and it is vital that the British government and those in other nations act now.
"In order to act and plan for the future it is essential that we have global estimates of how the number of people with dementia is going to change across countries with different patterns of ageing.
"We must make sure that sufficient funding and resources are available so that people with dementia and their carers get the care and support they need.
"Dementia must be taken seriously as a worldwide health problem."