When Janet Bostock retired in 2006 she was looking forward to spending more time with her two grandchildren, playing more tennis and getting an allotment.

Two years and three more grandchildren later, she spends two days a week looking after them, and much of the rest of her time looking after her 97-year-old mother. Recently she has also been trying to fit in some paid gardening work.

Janet, 63, is one of the 3.5 million grandparents in the UK whose mother or father is still alive. Last week, figures from the Office for National Statistics showed that the number of over-80s in Britain has doubled over the past three decades to 2.7 million. This increased life expectancy, and rising numbers of families with two working parents, has left many of today's grandparents “squeezed from all sides”. According to a study of people in their fifties and sixties by the Joseph Rowntree Foundation, one in ten now care for their grandchildren as well as their own elderly parents. The need to top up pensions with part-time jobs only adds to the pressure.

Janet has just returned home to southwest London from a two-night visit to her mother in Weymouth, Dorset. The day she left for Dorset, she had been looking after her 18-month-old grandson, Ed, in Surrey. The day before that, she looked after her other three grandchildren, Grace, 7, Joe, 5, and Frank, 3, in London. She made it back from Weymouth just in time for her book group. As we speak, her husband, Hugh, 64, is about to head off to Devon for a four-day visit to his 90-year-old mother.

How grandparents can make all the difference

Grandma and grandad play a key role in the raising of children. Two people share their experiences

BACKGROUND

• Families struggle to afford formal childcare

• Tackling the bad-parent society

• Granny flats: Housing your older relatives

• The new hip-op generation

“It's a constant round of planning where we are going next,” Janet says. “Our family responsibilities are greater now than ever before. You think you've retired - well, you must be joking. I hoped that I would be able to play a lot more tennis after retiring; in fact I am playing less than when I was working.”

Janet, who refers to herself as a “BEC” - both-end carer - says that looking after the grandchildren is, for herself and Hugh, a choice. However, she concedes that some grandparents feel they have no option.

“It is delightful looking after the grandchildren because we see them grow and develop. Looking after our parents is sad because we see them getting more dependent. I don't begrudge doing it, but with the time it takes up - on the phone, organising carers and travelling to see them and look after them - the pressure can really build up. That's when you feel like you are being squeezed from all sides.”

Forget the “sandwich generation”: those in middle age looking after their children and parents. Now it is pensioners, juggling the demands of three generations of family, who are feeling the strain.

Lynn Chesterton, the chief executive of the Grandparents' Association, says this multiple caring responsibility can have a huge impact. “You will find a lot of raised blood pressure and depression among people doing this; they only go to their GP when they can't cope any more. Families rarely talk about these problems until it's too late,” she says. “I suspect that a lot struggle on, because they don't want to let anyone down. If you had similar levels of stress at work, you would be off sick.”

Those who would have been in nursing homes ten years ago are often now in residential care homes, and others who would have been in residential care now stay in their own homes and rely on support from family members. Figures from the charity Carers UK demonstrate the scale of the issue: seven out of ten women and nearly six out of ten men will be carers at some point in their lives.

And the situation is likely to get more acute. Chesterton adds: “The current generation of retirees is probably the last that will be retiring on adequate pensions, whereas those in the middle of their careers now will not be so lucky. Add to that that we are now seeing the first generation of women who brought up their children as single parents coming up to retirement. The financial implications are even greater if you are on your own, as are the potential caring responsibilities.”

Katherine French, 60, looks after her grandchildren Ben, 2, and Nancy, five months, once a week while their parents are at work, and her 90-year-old mother who lives half an hour away. She works three days a week as an office administrator. Katherine asked for her name to be changed because, like many in her situation, she would be mortified if her family began to think of themselves as a burden.

“I take great pleasure in looking after my grandchildren as it gives me an opportunity to build up a relationship with them as well as helping my son and daughter-in-law,” she says. “It is tiring, but the joy received far outweighs this. ”

However, she hadn't envisaged that at this stage of her life she would be also playing such a key role in looking after her mother. Katherine does all her mother's shopping and takes her to doctors' appointments as well as anything else that needs sorting out. “It doesn't help that my relationship with my mother has never been brilliant. I do get tired with work and there is some emotional stress trying to fit all this in. But working enables us to feel financially secure, as our pension fund was depleted when my husband fell ill a few years ago.”

For some, work is a source of stimulation and structure. Barbara Young, 62, is about to start a new business as a life coach. She retired two years ago from her job as the head teacher of an independent primary school. At that time her mother's health was deteriorating rapidly and Barbara was doing her shopping, taking her to appointments and having her to stay every weekend. She also had four grandchildren who would come to stay for the odd weekend.

“Then my daughter had her second baby and suffered severe postnatal depression,” she says. “For the last few months of my mother's life, when she finally but broken-heartedly agreed to go into a nursing home, I was visiting her every day and spending a day and a night a week at my daughter's house 90 miles away looking after her and the children.”

Life-coaching provides a much-needed respite, and, of course, crucial extra funds. “I have worked full time all my adult life, except when my children were very small. Without work I would be bored stiff, and I have found something that will be both flexible and rewarding. It also means that I can afford to travel, which otherwise I wouldn't be able to do.”

At the Oxford Institute of Ageing, several studies have been undertaken on the pressures being levied on men and particularly women in their fifties and sixties in Britain's new multigenerational families. One study of 1,000 single mothers showed that, while 75 per cent of grandmothers had provided supplementary childcare for their grandchildren, very few wanted to do this full time or do more than pick them up after school, give them supper and oversee homework.

The problem, according to Professor Sarah Harper, who carried out the research, is that, as the Government emphasises that more care should be made available in the community, the family is again hit. Women in their fifties who want to remain in the labour market are being increasingly pulled in two directions, to look after their grandchildren and parents.

“It's difficult to say we're reaching crisis point, because people are very adaptable,” says Professor Harper. “But last year, we found in our study, The Future of Retirement, that people aged 60 to 79 years were contributing around £4 billion a year in voluntary work - and saving the country between £11 billion and £50 billion by looking after their families.

“Given the pensions crisis, these people often need to stay in the labour market to provide for their retirement, so the balance between work and care is becoming a real work-life question.”

The Grandparents' Association wants more recognition in the UK of the contribution grandparents make and more consideration, at policy level, of their needs. Two books out in September could be a sign that things are looking up. The Modern Grandparents' Guide (Piatkus, £10.99) explores the pressure modern-day grandparents are under, and The Really Useful Grandparents Book (Penguin, £18.99) suggests ways to occupy grandchildren.

“Maybe now it's not until your own children retire that you get a real chance to relax,” Janet Bostock says. “I hope I don't have to wait that long before I can put my feet up.”

Times recommends

• Live fashion chat: How to stay warm and chic
• How I lost the gift of joy for a while
• The A-Z of winter style rules