Age UK said there were more than a million older people who needed care but did not receive it from any source – be it family, friends, neighbours, or their local council.
The charity estimated that 300,000 of these people were also struggling to cope with loneliness much of the time.
Those who received some sort of care or support were much less likely to be lonely, the charity said.
Research has shown that loneliness not only has an adverse affect on quality of life, it can also make people more susceptible to illness.
Age UK has handed a petition to government calling for more action to combat loneliness among older people.
The charity said the Government must restore the “crumbling” social care system and fund more local support for lonely elderly people.
It also called on the public to find time for a friendly conversation with an elderly neighbour, phone the great-aunt they had not contacted for months and consider volunteering at Age UK or another charity that helped the elderly.
“It’s bad enough to be struggling because of a care need and going without any support, but now it turns out that an appreciable number of older people in this position are facing a double whammy because they are chronically lonely too,” Caroline Abrahams, charity director at Age UK, said.
“We think it is likely that many of these older people are living on their own and in quite isolating circumstances, unable to call on family, friends or neighbours for help if they need it.
“Frankly, this is no way to spend your days.
“The overriding purpose of social care is to meet a person’s social care needs but, of course, for an older person who can’t get out and about, a friendly chat – however brief – with a visiting care worker is extraordinarily precious if it’s the only conversation you’ll have all day.
“Our social care system is in decline and failing to keep pace with our growing older population, leading to more older people with care needs going without formal help.
“Now, we can see from our analysis that this is adding to the problem of acute loneliness among older people too.”
Izzi Seccombe, community and wellbeing spokeswoman at the Local Government Association, said: “Loneliness is a significant and growing concern for many older people and is something that is now being identified as a major public health issue.
“The impact of loneliness can be devastating and costly – with consequences comparable to smoking and obesity.
“This can be prevented with early intervention, which a number of councils are already successfully delivering in partnership with volunteer and community organisations.
“As our population profile changes, and we have a larger proportion of over 65s and over 85s, loneliness is becoming an increasingly important public health concern.”