More than 1.2 million people in England and Wales will have dementia by 2040, new research suggests.
People are living longer lives, which is fuelling a rise in the number with the condition, experts writing in the British Medical Journal (BMJ) said.
Researchers from University College London and the University of Liverpool used data to estimate that there were approximately 767,000 people with dementia in England and Wales last year.
They predict this figure will rise to 872,000 in 2020, 1,092,000 in 2030 and 1,205,000 in 2040.
As a result, there will be a 57% rise in cases in 2040, compared with last year, they said.
The Alzheimer's Society, which uses figures for the whole of the UK, has predicted that dementia cases will soar to two million by 2051.
According to its data, 225,000 will develop dementia in the next year - one every three minutes.
Current costs of dementia to the UK economy are estimated to be £23 billion.
The research shows that although the number of newly diagnosed cases of dementia is falling, the overall number of people living with the condition is set to increase, largely due to longer life expectancy.
Experts used data from 18,000 men and women from the English Longitudinal Study of Ageing, tracking the health of those aged 50 and over.
From 2002 to 2013, people were selected at random at six different points. They were tested for memory, verbal fluency and numeracy function, and basic activities of daily living such as getting in or out of bed, dressing and eating.
Dementia was then identified by these assessments, together with interviews with carers, or through official NHS diagnosis.
Dr Sara Ahmadi-Abhari, lead author from University College London, said: "The risk of developing dementia at any given age is going down over time, shifting dementia to later years in life.
"This decline is mainly because of improvements in healthcare and adopting healthier lifestyles.
"Our estimate of 1.2 million people with dementia by 2040 is based on the assumption that the decline in risk of developing dementia continues to the future.
"If public health efforts fail and the risk of developing dementia does not continue to decline, the growth in numbers of people living with dementia will be much larger, reaching 1.9 million by 2040."
Dr James Pickett, head of research at Alzheimer's Society, said: "With an ageing population and no way to cure, prevent or slow down the condition, dementia is set to be the 21st century's biggest killer.
"These latest estimates are yet another wake-up call that the current social care system, already on its knees from decades of underfunding, needs urgent attention from the Government if it's to cope with the inevitable massive increase in demand.
"Researchers must unite to achieve breakthroughs in prevention, treatment and care before dementia becomes an even larger health and social care crisis."