Millions of unpaid carers are spending more than 100 hours a week looking after loved ones.
Almost four in 10 of 57,000 surveyed in the UK have that workload, the Health and Social Care Information Centre found.
It also found 17% did the job with no support and 15% neglected themselves by not eating or sleeping well.
Carer campaigners said overburdened carers were being left exhausted and isolated. There are seven million carers in the UK.
The shock revelation comes as new figures show adult social care cash has been cut by 3% in real terms.
Campaigners blasted councils and the Government and urged them to do more to help “suffering” carers.
Helena Herklots, chief executive of Carers UK, warned that growing numbers of families were “providing more care with less support”.
She said thousands of unpaid carers were “finding it harder to look after their own health, maintain relationships with others and have a life of their own alongside caring”.
She added: “Carers save the economy £119billion every year in the unpaid care they provide, yet their own health and wellbeing is suffering.
“As the Treasury finalises the spending review, the results of the survey should be a wake-up call that more funding is needed for support to back-up carers.”
The HSCIC survey of carers found almost one in five – 17% – had no support at all.
HSCIC statistician Katharine Robbins said: “This survey helps lift the lid on the lives of the many thousands of people who care for another adult as part of their everyday life.
“It shows a wide range of experiences of carers known to local authorities.”
Separate HSCIC figures found a fall in spending by councils, from £17.2billion in 2013/14 to £17.1billion in 2014/15. This was a 3% drop in real terms.
Nearest tube – Elephant & Castle underground station (Northern and Bakerloo lines).
Nearest Railway Station – Elephant & Castle
Buses from Elephant and Castle – ask bus driver for Burgess Park. Bus numbers: 12, 171, 148, 176, 68, 484, 42, 40, 45