Some 60% of people claiming the main long-term sickness benefit believe the amount of money they get is not enough to live on, according to a major new survey.
Labour called the findings from Employment and Support Allowance (ESA) claimants "a damning indictment of the Government's treatment of sick and disabled people".
Campaigners warned that claimants were struggling to stay healthy, pay bills or afford to eat even before further cuts to ESA came into effect last year.
The Department for Work and Pensions said it did not recognise the findings and its own research showed 83% of people were satisfied with the benefit.
The online survey was carried out by the Disability Benefits Consortium, a national coalition of more than 80 different charities and other organisations.
Analysis of the results shows 60% of 1,755 respondents said the amount of ESA they receive is not enough to live on.
When asked about the consequences of this, 62% said they struggled to stay healthy, 49% said they could not pay bills, 36% could not afford taxis to medical appointments, and 32% said they could not afford to eat.
"This is a damning indictment of the Government's treatment of sick and disabled people," said Debbie Abrahams, the shadow work and pensions secretary.
"It is shocking that people on ESA are struggling to afford the basics and having to make a choice between heating or eating.
"If this Government is committed to a fairer society, they should stop trying to rebuild the economy off the backs of poor, sick and disabled people."
ESA is the main long-term sickness benefit, paying 2.4 million people a minimum of £73.10 a week.
Last April, ministers introduced a £30-a-week cut for new claimants in the so-called ESA Wrag group - those who had been found not fit for work, but who may be in the future.
Kevin Stannard, 62, from Colchester, was made redundant from a blind-fitting firm he worked for for 40 years in 2010 after his Parkinson's symptoms stopped him working.
He and his wife Amanda said they faced a "horrendous" four years in the ESA Wrag group before being moved into the support group in 2016.
"We couldn't afford to pay bills or buy food, so we were forced to live off credit cards the whole time and we're still paying them off now," Mrs Stannard said.
"It got to the point that we were going to lose the roof over our heads as we couldn't pay the rent. Luckily our local councillor stepped in to help us at that point.
"The whole process of ESA is so confusing. I took it on because it was too difficult for Kevin to deal with alongside his Parkinson's.
"But the stress of it all led to me having a stroke, which means I've also had to give up work, and I noticed a decline in Kevin's health too.
"We were never informed of any changes to the support that Kevin would receive so it came as a complete shock and, despite in the end the Department for Work and Pensions agreeing that we were right, we are still living with the consequences of that awful time."
Four in five claimants told the DBC survey their "fit for work" tests made their health worse due to stress and anxiety, while 62% thought their assessor did not understand their condition.
Around two-thirds of 3,530 respondents said their experience of the disability benefits system was "bad" or "very bad".
The consortium is due to meet Disabilities Minister Sarah Newton to raise these issues this week.
They will also urge the Government not to scrap the support group and accept that some people will never be able to return to work.
"It is unacceptable that under ESA people are living with the stress of not knowing if they can afford to stay healthy, eat or pay bills," said Phil Reynolds, a DBC co-chairman and senior policy and campaigns adviser at Parkinson's UK.
"Even more worryingly, these results are from people on the previous rate of ESA. Due to cuts, new applicants from April 2017 will receive a third less money, which will inevitably make it even more difficult to make ends meet.
"This paints a dire picture for disabled people who will also be at risk from the roll-out of Universal Credit in 2018."
A DWP spokesman said: "We do not recognise these figures.
"The latest official research shows that 83% of ESA claimants are satisfied with their overall experience."
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