Plans to shake up incapacity benefits will fail unless changes are made to the way people are assessed, the Citizens Advice Bureau has warned.
The Citizens Advice Bureau has called for assessment changes
It believes thousands of people who are sick or have disabilities are being wrongly denied benefit each year.
In recent years, more than 60% of appeals against incapacity benefit refusal have been successful.
But the Department of Work and Pensions has said its own findings show that the number of those dissatisfied was small.
The Citizens Advice report was based on hundreds of complaints to the charity.
Some claimants complained doctors that assessed them were insensitive or rude.
People with mental health problems are particularly likely to be wrongly assessed, according to the report's authors.
In one case a woman with severe mental health problems lost her benefit after an examining doctor said she "didn't look mental", the report said.
She had to live on £39 a week for six months while awaiting an appeal.
The report called for the "deeply flawed" decision-making process to be improved.
It said more than 60% of decisions were overturned on appeal because there were "so many wrong decisions to refuse or withdraw benefit".
The report largely blamed the way medical assessments were carried out and the weight they were given.
It criticised the department for relying too heavily on the assessment results, rather than evidence from claimants' own GPs or other practitioners.
The department said "unhappy" clients tended to ask the Citizen Advice Bureau for help, whereas those that were satisfied with the handling of their claim did not comment.
"Put into context of nearly one million medical examinations a year, customer satisfaction surveys show the proportion of clients who are dissatisfied is small compared to those who feel they have received a satisfactory service," the department said.
It said it was "continually improving its standards of decision making and customer service to benefit claimants".
A spokeswoman emphasised that the department expected to address problems with assessments.
This was to ensure people were "not left to a lifetime of benefit, but are given the help and support they need to enable them to return to work."