Almost a third of older men who have long-term health problems are lonely, according to a new report from Age UK.
Over the next 14 years, the charity warned that there will be a 65% rise in the number of older men living alone, which will greatly increase their risk of becoming lonely.
Age UK has now called for urgent action to tackle what is fast becoming a "major public health challenge".
Caroline Abrahams, charity director at Age UK, said: "Loneliness is often a hidden issue, as many older men tend to be stoical and reluctant to admit how lonely they are – but facing the ups and downs of later life alone shouldn’t be ‘the new normal’ for any older person."
Evidence shows that older men who live alone are more likely to be lonely than their female counterparts and to have less regular contact with family and friends, which exacerbates feelings of loneliness.
According to Age UK, the risk of loneliness and isolation appears to increase with age, and among those with long-term health problems or disability.
The charity has warned that although as a society we are becoming more aware of the risks of loneliness to older women, we need to understand that older men can also be badly affected and that the numbers of lonely older men are substantial and on the rise.
One of the main reasons is that life expectancy for men is catching up with that for women.
In a survey by the charity, almost 90% of over-65s felt there should be more help available for lonely older people, with well over a quarter believing that "a simple visit once a week" would help most people who feel lonely.
A further 15% believe that a local club or group, or getting to know their neighbours better, would most help.
"Loneliness is a widespread problem among older men, especially for those who are unwell, bereaved or who have seen family and friends move away," said Caroline Abrahams from the charity.
"As more older men live longer we need to appreciate that the numbers who are chronically lonely are likely to increase too – unless we do something about it, which we can and we must do."
Loneliness has a serious impact on our health, which can lead to greater reliance on health and social care services.
Around one million older people are "chronically lonely" at any given time in the UK. This has implications for people’s mental health - in fact research shows that it can be as harmful as smoking 15 cigarettes a day.
It also increases the risk of conditions including dementia, high blood pressure and depression.
The charity believes there has been an increase in loneliness among older people as families are more geographically scattered; working families live busy and hectic lives; and as more of us use electronic devices, older people risk feeling very left out if they can’t join in.
To tackle this issue, more steps should be taken to identify and support older men and women who are lonely and isolated, said the charity.
It is calling on the government to recognise loneliness and isolation in later life as a serious health problem and commit to action to help tackle it.
Charlotte Abrahams said: "We all have a responsibility to take action and help the older people in our lives. Father’s Day is a great opportunity to re-connect with older relatives, and this year we especially want to draw attention to the needs of older men, particularly if they are unwell or live alone.
"A simple phone call or a visit could mean the world to someone who is feeling lonely and forgotten."