Another 2.5 million people in England will be living with major illnesses such as cancer, diabetes, dementia and depression by 2040, according to a report.
Most of the increase will be down to ageing, with more people living longer and contracting illnesses.
The forecast rise will come as there is also only a relatively small increase in the working age population.
Added together that means more people needing treatment while fewer people are there to pay the taxes that fund the National Health Service.
The study from the Health Foundation found that 9.1 million people in England will have a major illness – an increase of 37 per cent from 2019.
“Almost one in five of the population are projected to be living with major illness by then, an increase of more than a third,” the report said.
“By comparison, the working age population is projected to increase by only 4 per cent.
"This population group will be responsible for generating the bulk of government revenues used to fund public services, including the NHS.”
Cases of dementia are expected to rise 45 per cent per cent by 2040, heart failure by 92 per cent, cancer by 31 per cent, diabetes by 49 per cent, chronic pain by 32 per cent, and anxiety or depression by 16 per cent.
Only one out of 20 conditions listed – coronary heart disease – is expected to drop, because of declining smoking rates and the use of medications such as statins.
At 70, people will have an average of three long-term conditions, rising to more than five by the age of 85, researchers said.
They said four fifths of the jump in major illnesses will be driven by an ageing population, with people living longer meaning they are more likely to encounter – and live with – ill health.
About 80 per cent of the projected increase in major illness will affect those aged 70 and over.
The study suggested that some gains, such as fewer people smoking and lower cholesterol rates, will be offset by the effect of obesity as many people who have been obese for long periods of their lives then reach old age.
But tackling obesity, which is a major risk factor for many of the conditions described, could have an effect in the future, the experts said.
Anita Charlesworth, director of the Real Centre at the Health Foundation, said that, over the past 30 years, obesity levels in the adult population have broadly doubled.
“Some of this is almost certainly baked in and very difficult to change but, clearly when you look at things like Type 2 diabetes, if people were to reduce their weight significantly after diagnosis of Type 2, that can make a big difference to quality of life," Ms Charlesworth said.
She said that, while the report looks at adults, there are “really concerning obesity rates” among children, which will have a big future effect on the NHS.
Ms Charlesworth said that, at present, 6 per cent of the working age population are out of the labour market because of ill health but this will rise by 500,000 people.
“There is no silver bullet to reduce the growth in the number of people living with major illness," the report said.
“A long-term plan is needed to reform, modernise and invest in the NHS, alongside a bold new approach to investing in the nation’s health and well-being.”
For the study, researchers used health and death records to look carefully at the 20 health conditions that account for 65 per cent of the burden of illness in England.
They also noted that life expectancy by 2040 is expected to rise to 83.1 years.
