The UK's "army of unpaid carers" is being "let down" by the failing social care system, campaigners warn.
Eight leading charities, including Age UK and Carers UK, have joined together to highlight the plight they face under the umbrella of Carers Week.
Feedback from 3,400 carers showed those providing the most intensive care often ended up with health problems themselves.
The government said there was now extra funding to allow carers' breaks.
It is estimated that one in six carers either give up work or reduce their hours to look after elderly friends and relatives or younger adults with disabilities.
The charities said it demonstrated the hidden effect of the squeeze on council-run social care support.
The numbers of elderly and younger adults getting help from councils has fallen in recent years.
And the coalition of charities said it was the 6.4m unpaid carers who were being left to take up the strain.
It asked carers what impact caring and the lack of support had on their health. Most of the responses to the self-selected poll were from those heavily involved in caring for someone, often providing support on a daily basis.
Eight in 10 said their responsibilities had caused them health problems, including everything from back pain to insomnia.
And two fifths said they had delayed seeking medical help with some reporting that had had serious consequences, such as delayed cancer diagnoses and damage to lungs.
The coalition said carers should be offered regular health checks and there needed to be better information and support available.
Carers Week manager Helen Clarke said: "It's a scandal that carers can't get the time or support they need to look after themselves which could be jeopardising their health as a result.
"Carers are feeling the strain of a woefully underfunded system and still we're seeing more cuts.
"Unpaid carers save the Government a fortune - £119 billion a year - yet they're let down in return. It is time for urgent action to tackle the crisis in social care."
This is just the latest research to highlight the problems facing the social care system.
Councils themselves acknowledge they are struggling to meet demand.
Last week a survey of social care chiefs showed that cuts were still being made despite the government pumping extra money into the system.
Sarah Pickup, president of the Association of Directors of Adult Social Services, said: "It could not be clearer that there is a desperate need for politicians from all parties to quickly find an answer to how we, as a nation, are going to adequately fund social care services in the future."
Ministers have promised the system will be reformed.
A Department of Health spokesperson said £400m of additional NHS funding had been allocated until 2015 to pay for carers' breaks.
She added: "We know how important it is for carers to be able to take a break from their caring responsibilities, to protect their own physical and mental health.
"We also want to place the rights of carers on a much firmer footing, so that the law recognises carers' rights and their role in caring for others."
A social care white paper is expected to be published in England in the coming weeks, but there are fears the trickiest issue - how to fund it - will be put off.