Dianne Jackson, manager of Stirling Council on Disability (CoD), said the voluntary organisation had seen a 50% increase in requests for help in the last year, with numbers continuing to rise sharply.
Many disabled people are being assessed as fit for work, regardless of their condition or the effect it has on them, she said. They are often advised to raise an appeal and get a medical report from their GP to support their case. But most GP practices charge at least £10 for this service, and often much more.
"The minimum charge a patient may be expected to pay for evidence of their conditions is £10; our organisation has seen charges in excess of £75. For most people existing on benefits this is exorbitant and often impossible," she said.
"Some GP practices are refusing to provide it, under the assumption that the Department of Work and Pensions (DWP) will request it if needed."
Stirling CoD is not criticising GPs for overcharging, she added. "The onus should be placed on the Government's welfare policy, which did not appear to anticipate the extra workload and cost to practices in providing the records."
The charity was visited 663 times between April 2011 and December 2011. Between April 2012 and December 2012 the figure was 995, with the rise driven in large part by requests for support with applications and tribunal hearings. Its workload in the new year includes a large amount of tribunal work, even before the staged introduction of the new Personal Independence Payments, throughout 2013, Ms Jackson said.
"With good preparation and evidence, there is a better opportunity to reinstate benefits for those entitled to them," she said. "We are fighting what is tantamount to a war, for the rights of those less able.
"Five people in the Stirling area are chasing every job vacancy, and these presumably are jobs for those without disability. Realistically what hope is there for those with a disability?"
She criticised the assessments carried out on behalf of the DWP by the company ATOS, and added: "The assessment bar is set so low that the majority of people are assessed as fit for work regardless of their conditions. These assessments are carried out by doctors, registered nurses, or 'disability analysts'. The average timespan of assessment is 30 to 40 minutes.
"The evidence gained, by constant reference to a computer programme, is used by DWP decision-makers and very frequently overturns long-standing evidence provided by GPs and consultants. GPs may have known a client for a number of years." It is not clear what acknowledgement or respect is given to the views of a claimant's family doctor, she said.
She added: "It is becoming increasingly difficult for appellants to gain access to their medical reports from GPs and consultants. Did the Government give any consideration to the extra pressure upon practices to provide evidence? I don't think so."
Dr Alan McDevitt, chairman of the BMA's Scottish GP committee, said welfare reform was likely to have a significant impact on general practices, especially those with a high proportion of patients in receipt of the former incapacity benefit. "There has been a dramatic increase in the numbers being assessed as fit to work and a massive number of appeals made against these decisions," he said. "Our patients are very concerned and confused with regards to these assessments. Evidence appears to suggest people with serious health conditions are frequently declared fit for work.
"The system of benefits is highly complex and needs to be simplified. However, these assessments and the subsequent appeals process can have a devastating effect on our patients' mental and physical health."
A spokeswoman for the DWP declined to comment on the fees charged by GPs for medical assessments.
She said GPs were required to provide an assessment to the department at no cost, where the department requires it.