Loneliness is 'major health issue'
"No man is an island, entire of itself; every man is a piece of the continent, a part of the main," wrote the poet John Donne in the 17th century, in an expression of loneliness and longing.
Some people seek isolation, but few choose to be lonely, primarily because it isn't good for us.
Loneliness doesn't just make people unhappy, research shows that it has an effect on mortality too.
It is also associated with poor mental health and, more surprisingly, with conditions such as cardiovascular disease, hypertension and dementia.
Loneliness is a public health issue that should be tackled urgently, says Laura Ferguson, director of the Campaign to End Loneliness, a coalition of organisations working to combat the problem including Age UK.
"There are links with early death. The risk factor is similar to smoking and worse than obesity."
With the prospect of an ageing population growing in size in years to come, it seems inevitable that loneliness can only become an even bigger problem.
'Social high point'
David McCullough, chief executive of WRVS, formerly the Women's Royal Voluntary Service, which has 40,000 volunteers supporting older people across the UK, says loneliness is widespread.
"We deal with a frail, elderly population who suffer from infirmity and loss of mobility.
"When I go on meals on wheels runs it bring it home to me. There are all these people waiting at the door for us, for a visit by another human being. It's the social high point of their day."
Research carried out over the last few decades has consistently shown that 10% of older people always feel lonely or feel very lonely.
Recent estimates place the number of people aged over 65 who are often or always lonely at over one million.
The Campaign to End Loneliness says that half of all older people, about 5 million, say the television is their main company.
Yet the 10% figure has remained roughly stable for the past 60 years.
'Quality of life'
Prof Christina Victor, from Brunel University, who has done lots of research into loneliness in later life, says this is surprising.
"There is no robust evidence that the problem is any bigger now than it was, despite the change in family structures and more people living alone."
But she maintains that reducing loneliness would be beneficial to health.
"One major predictor of quality of life is good, social relationships."
Loneliness doesn't just affect the over 65s, however. Younger people are prone to it too, particularly 18 to 24 year olds.
While an older person's loneliness might be triggered by poor health or the loss of a partner, a younger person's sense of emptiness could coincide with becoming unemployed, leaving home or having a child.
"Loneliness is the difference between your desired contact with people and the contact with people you actually have," says Prof Vanessa Burholt, from the Centre of Innovative Ageing at Swansea University.
"This explains why some people with lots of friends still feel lonely. It's a subjective thing."
Prof Burholt's research into the pathways to loneliness suggests that our environment and our mental health can have an impact on perceptions of social relationships.
"Depressed people find it harder to change their perception of the level of personal relationships they need. They are not able to adjust it."
The Campaign to End Loneliness has launched a new digital toolkit to help local councils and health authorities tackle loneliness in their area. It is funded by the Department of Health.
The toolkit contains the latest health research on loneliness, ways of collecting data on loneliness, advice on monitoring and treating loneliness and scales to measure it.
Care Services Minister Paul Burstow said society needed to do more about the issue.
"Loneliness can have a significant impact on people's health, yet, unlike risks such as alcohol and obesity, it is still out of sight."
The good news is that people can recover from loneliness. It isn't a life-long condition. People tend to move in and out of loneliness at different stages of their lives.
Befriending services can work for some older people who are lonely while a simple friendship with a neighbour or local volunteer can work for others.
People with depression are more likely to need a different approach, such as cognitive behavioural therapy, says Prof Burholt.
It would be a mistake to think every older person is lonely.
"Throughout life there are peaks and troughs," Prof Burholt says.
"We are constantly negotiating what our social resources are and whether we feel lonely or not."