A third of these will be diagnosed with dementia, depression or a cognitive impairment.
Within the next 20 years British men and women over 65 are expected to live 3.6 years and 2.9 years longer than older Britons now.
However there will also be a massive expansion in the number of people suffering from multiple diseases, known as multi-morbidity.
Men will live on average of 18.6 years and women 21.2 years past their 65th birthday.
But five-and-a-half years of the extra years in men and five years in women will be spent with multi-morbidity and two-thirds or more of the gain in life expectancy will be spent with four or more diseases.
And Britons under 65 who are overweight or obese and do little exercise are a ticking time bomb and the increased illnesses poses a challenge to the NHS and social care, Newcastle University’s Institute for Ageing warned.
Professor of Epidemiology of Ageing Carol Jagger explained: “Healthcare delivery was built, and generally remains centred, on the treatment of single diseases.
“Over the last decade, the growing number of older people aged 65 years and over has become a considerable challenge to health and social care service provision and funding, as over 50 per cent have at least two chronic conditions – multi-morbidity.
“Moreover numbers of the very old, aged 85 years and over, are set to double over the next 20 years, with multi-morbidity the norm in this age group.
“Multi-morbidity increases the likelihood of hospital admission, length of stay and readmission, raises healthcare costs, reduces quality of life, and increases dependency, polypharmacy and mortality.
“In addition to multi-morbidity, many of the very old have sensory impairment and incontinence, making a single disease-focused model of healthcare unsuitable.
“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 multi-morbidity
“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.”
So the new study looked at how key long-term conditions and multi-morbidity will evolve between 2015 and 2035 in the over 65s in England.
It used a new dynamic microsimulation model, Population Ageing and Care Simulation (PACSim).
It stimulates the survival and characteristics for example disease and associated risk factors of a set of real individuals over 35 as they age over time, to estimate future prevalence, incidence, and life and health expectancies.
It found over the next 20 years the largest increase in diagnoses will be cancer up by 179.4 per cent and diabetes up by 118.1 per cent in the older population, whilst arthritis and cancer will see the greatest rise in prevalence of 14 and 15.1 percentage points respectively.
The exceptions are coronary heart disease (22.1 per cent), depression (15.1 per cent) and cognitive impairment other than dementia (25.6 per cent).
In the over 85s, all diseases apart from dementia and depression will more than double in absolute numbers between 2015 and 2035.
Prof Jagger said: “Much of the increase in four or more diseases, which we term complex multi-morbidity, is a result of the growth in the population aged 85 years and over.
“More worryingly, our model shows that future young-old adults, aged 65 to 74 years, are more likely to have two or three diseases than in the past.
“This is due to their higher prevalence of obesity and physical inactivity which are risk factors for multiple diseases.”
Professor Jagger adds: “These findings have enormous implications for how we should consider the structure and resources for the NHS in the future.
“Multi-morbidity increases the likelihood of hospital admission and a longer stay, along with a higher rate of readmission, and these factors will continue to contribute to crises in the NHS.
She concluded: “Our findings indicate that over the next 20 years, the extra years spent with multi-morbidity after age 65 will exceed the gains in life expectancy with an expansion of morbidity, resulting in greater primary and secondary healthcare utilisation.
“There is also a pressing need to consider the implications for social care, both from formal care services and from unpaid family and other carers, given the strong association between multi-morbidity and reduced functional capacity.”
The study was published in Age and Ageing, the scientific journal of the British Geriatrics Society.