More than seven million people in the UK are providing unpaid care for family members, a report has said.
Almost 15% of adults now give up their time for free to look after relatives, providing an average of 19.5 hours of care per week, independent think-tank Social Market Foundation said.
The study found that the number of family carers in Britain - 7.6 million - had risen by a million over the course of a decade, while the proportion of those providing 20 or more hours of help a week had increased by 4% between 2005 and 2015.
Age UK, which sponsored the research, warned more support was needed for so-called informal carers, and urged the Government to direct more funding to the care system.
Charity director Caroline Abrahams said: "The informal carers we meet at Age UK are usually deeply committed but they also often tell us that they are completely exhausted. Not only do they need more opportunities to take a break, they also require the back up of a reliable, effective social care system.
"When informal carers are trying to cope all on their own, this is a recipe for burn out and the collapse of an arrangement which could otherwise work really well for an older person.
"It would be dangerously complacent for policymakers to assume there is an infinite supply of wonderful people able and willing to provide informal care for their loved ones. For all kinds of demographic, social and economic reasons our current system of social care is living on borrowed time - it urgently needs transforming and in 2018 we must make a serious start."
The report, which comes as the Government prepares to publish its green paper on social care, said family carers provided 149 million hours of care each week - equal to the work of four million full-time paid care-givers.
More than half (59%) of those caring for an elderly relative are women, while 19% of women in professional jobs provide care - up from 18% in 2005, the study added.
Caring can also have an impact on earnings, the report said, as people are more likely to work less than those without caring responsibilities.
Kathryn Petrie, Social Market Foundation economist, said: "More women with professional and managerial jobs are trying to combine work with family care.
"We know that carers are often driven to reduce their hours or leave work altogether and, without proper support for these carers, there is a risk that women are increasingly driven out of professional careers, reversing recent progress towards equality in the workforce."
The Department of Health and Social Care said its forthcoming social care green paper would look at long-term sustainable solutions for the social care system.
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