The director of Bristol's Citizens Advice Bureau has said more people are appealing against decisions to move them from incapacity benefits to jobseekers allowance.
Ken Pickering said he first noticed the trend between April and June.
He said that earlier this year, claimants had to undergo a test to assess if they were capable of work.
The Department for Work and Pensions said it was unfair to suggest the system was not working.
In the UK, more than two million people are on incapacity benefit or its successor, employment support allowance, costing about £12.5bn a year.
The government has said it will reassess all claimants of incapacity benefit for their "readiness to work".
The extent of the welfare cuts will be announced in the Spending Review later this month.
A Department for Work and Pensions spokeswoman said there were 17,870 people on incapacity benefit in Bristol.
Mr Pickering said that between 1 April and 30 September the bureau dealt with 499 inquiries regarding employment support allowance.
He said no figures were available about how many were appeals.
Mr Pickering said: "Quite a large percentage of people assessed found they were moved from employment support allowance on to jobseekers allowance, which meant they had quite a fall in income in some cases.
"A number came to us for help with appealing."
He said the number of successful appeals had also risen.
"The trends did suggest people were being affected by the benefits change, he said.
"Of those that did appeal, over a third were found to have been badly assessed."
He said the loss of benefits would have knock-on effects for legal aid centres, the local economy and the NHS, as the loss led to health problems including depression and anxiety in some people.
In June, the secretary of state appointed Professor Malcolm Harrington to undertake an independent review and prepare a report.
A Department for Work and Pensions spokeswoman said: "It's unfair to suggest that the system isn't working - of all new claims to ESA, only 5% have had a fit for work assessment overturned by the Tribunal Service to date.
"However, we always want to improve, which is why Professor Harrington is leading a review into whether we need to make any further changes to the assessment.
"This review includes a number of the charities representing disabled people."