Researchers have put a financial price on an "epidemic of loneliness" - estimating it costs £6,000 per person in health costs and pressure on local services.
But the London School of Economics study of older people says for every £1 spent in preventing loneliness there are £3 of savings.
Deborah Moggach, author of the novel adapted for The Best Exotic Marigold Hotel films about retired people from the UK going to India, said: "Loneliness really is the last taboo."
She said old age must not be an "inevitable descent into despair" and more efforts had to be made to stop people becoming isolated and lonely.
"We have to stop thinking of this as someone else's problem. As a society, we need to recognise loneliness as an issue, and put something in place that enables older generations to flourish - not flounder," said Moggach.
'Stigma' of loneliness
The research from the LSE has been published alongside the Campaign to End Loneliness, a group set up by charities and local authorities.
There are an estimated 1.2 million people in the UK who have "chronic" loneliness, with links to poorer physical and mental health and increased use of GPs, hospitals and social services.
The campaign says it should be seen as a problem in public health terms, like obesity and smoking.
For a decade of an older person's life, the extra economic cost of loneliness is calculated as £6,000.
Over five years, preventing loneliness could see a saving of £3.6m, say the researchers.
Loneliness is also linked in the study to earlier death and higher risks of dementia.
The study says there have been a variety of attempts to tackle loneliness - such as befriending schemes, healthy living initiatives and better "signposting" to show the elderly where they can get help or meet other people.
This week, Guides in Manchester launched a new badge to tackle loneliness in old people, with young people visiting the elderly.
'They're closed off'
But the LSE research also shows that there is a "stigma" around admitting to loneliness.
"It implies that there's something wrong with us," said Moggach.
"We talk about everything else - death, sex and money."
She said the proliferation of technology could make people feel even more isolated in public places.
"Everyone's on their phones, they're closed off."
She said the "last bastion" for old people of chatting to people at the supermarket checkout was being taken away by the increase in automated machines.
People in later life could become "hugely cut off," she said, describing the loss of friends and independence as "chronic, low-level bereavement".
Laura Alcock-Ferguson, executive director of the Campaign to End Loneliness, said: "There is much to do to overcome loneliness.
"The huge stigma surrounding it is clear, which is slowing down efforts to combat it. This is isolating millions of older people - and with our ageing population, the epidemic of loneliness is growing fast."
"It can be hard," says June, a 76-year-old widow living near Peterborough. "I have to push myself."
She looks forward to a tea party once a week, on a Sunday, which she says can feel "like the loneliest day of the week".
Loneliness can be "hard to put in words", particularly when she is in touch with family and neighbours.
She has ways to keep in contact - "my iPad is my friend" - and likes to talk to people in Facetime video calls.
But she says she wishes there were more places for elderly people such as herself to go to have some company.
She has volunteered with a local church project making meals for people in need, such as the homeless, and she likes the sense of giving something back.
But she misses the lack of places to go and chat to people - made worse by local shops and post offices shutting down.
And there are other older people she worries are even more isolated and who might go days and weeks without talking to anyone.