Within 20 years more than two million Britons will be living with four or more chronic illnesses due to years spent living a sedentary lifestyle, new research has found.
Although life expectancy is set to increase by 3.6 years for men and 2.9 years for women, two-thirds of this extra time is likely to be marred by disease.
The study, published in the journal Age And Ageing, found that a third of those suffering from four or more diseases would have mental health problems such as depression or dementia.
In the UK, the number of people aged above 85 is set to double from 1.3 million in the next 20 years.
The report's authors said there was now a "pressing need" to consider the implications on social care because of the link between multiple chronic diseases and reduced functional capacity.
A study of 303,589 individuals aged 35 years and over found that 54% of people above 65 in 2015 already had two or more chronic illnesses.
It predicted that by 2035, 2.5 million, or 17% of people over 65 would have four or more chronic illnesses, while 67% of this group would be living with two or more.
"Poor health behaviours such as obesity and physical inactivity are risk factors common to a number of diseases, but have received little attention as risk factors for multimorbidity," the report said.
"Younger cohorts have a higher prevalence of obesity than their equivalents a generation ago, which may contribute to the increased prevalence of multi-morbidity in those under 65 years of age."
It urged the government and health service to focus on mid- and later-life risk factors such as obesity and lack of exercise, as well as earlier intervention when older people acquire their second chronic condition.
It also called for more efficient health and social care to meet the needs of people with four or more long-term conditions.
Professor Carol Jagger, professor of epidemiology of ageing at Newcastle University and one the report's authors, told the Times: "It is primarily the lifestyle factors that are driving these diseases."
She added: "We are much more sedentary than we were before."