There has been an "alarming" rise in the number of elderly people admitted to hospital with life-threatening infections due to failings in GP and community care.
A damning report from Age UK also found that more than a million people are left struggling with basic tasks such as eating and washing because of cuts to social care.
Around one in five hospital admissions in England are for illnesses that could be managed outside hospital with effective treatment and ongoing care by GPs, nurses and social care teams.
But figures show cases are rising, with hospital admissions among people aged 60 and over for pneumonia more than doubling between 2005/06 and 2013/14.
The number of hospital admissions per 100,000 people aged 60 and over for suspected urinary tract infections rose by 81% during the same period, while there was a 43% rise in admissions for congestive heart failure.
Cases of gastroenteritis rose by a third while one in four admissions for long-term conditions that could possibly be managed in the community were for chronic obstructive pulmonary disease (COPD).
Across all such conditions, there was a 48% rise in emergency admissions between 2001 and 2013.
All these conditions cost the NHS £1.4 billion a year, with those aged over 75 accounting for 40% of the total spend, the study said.
The report - examining the breadth of health and social care for older people - also found rising numbers of elderly people receiving no help from anyone.
It found hundreds of thousands of people suffering difficulty undertaking tasks such as getting dressed, washing, eating, managing medication, cooking and shopping.
The report said: "More than a million older people in England now have at least one unmet need for social care, compared to 800,000 in 2010; this means they receive no help from their local authority or from family, neighbours or friends.
"Around half of those older people who have difficulty bathing or getting dressed without help, and around a third of those older people who have difficulty going to the toilet unaided, are receiving no support."
The study also found that spending on social care has fallen "quite spectacularly" over the last five years.
In 2005/06, more than 1.2 million older people were receiving social care support from social service departments. By 2013/14 that number had fallen by around 377,000 to just over 850,000.
Caroline Abrahams, charity director at Age UK, said: "The rise in hospital admissions due to infections such as pneumonia is troubling but not altogether surprising, given the severe pressures on GP practices and community health services.
"Social care services are in sharp decline and growth in the older population has been outstripping investment in these parts of the NHS that help older people at home.
"Starving social care of resources is seriously undermining the operation of the NHS - our hospitals especially.
"Too many older people are going without the support they need and are being left to struggle on alone, sometimes ending up in hospital when this could have been avoided and exacerbating a destructive vicious circle."
Tom Gentry, policy advisor at Age UK, said elderly people often ended up on too many medicines and should be reviewed regularly by their GP.
"GPs historically have not done it very well," he said.
"You are getting to the point when people are on 10,15 medications and they are going into hospital and new medicines are being added.
"It reduces their overall resilience. There is very strong evidence that over a certain threshold these medicines are not doing you any good anymore and they are raising toxicity, reducing your balance and reducing your ability to walk about."
A Department of Health spokeswoman said: "In the period since these figures, we've taken significant action to ensure our ageing population gets necessary care.
"All over-75s should now have a named GP responsible for overseeing their care to help prevent them ending up in hospital unnecessarily, and we've set up a £5.3 billion fund that joins up health and social services.
"Going forward, we're giving the NHS an additional £10 billion during this Parliament to deal with extra demand."
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