The government is "not learning from its mistakes" in benefit assessments, according to disability charity Scope.
The National Audit Office today released a new report questioning the way in which 738,000 face-to-face medical tests on benefit claimants have been delivered by private company Atos.
Earlier this month the government announced that Atos and Capita won the contracts to run a new work-capability check for disabled people.
The NAO said DWP had failed to penalise Atos for "underperformance" or set appropriate targets.
Richard Hawkes, chief executive of Scope, said:
"The government and Atos have come under a great deal of criticism about how the fitness for work test is being delivered to disabled people.
"High rates of appeals and alarming evidence from the way assessors are trained to deliver the test do little to reassure disabled people that decisions about them are being made in a fair and appropriate way.
"Disabled people want to work but they face a multitude of barriers to actually finding a job. Inaccessible transport, a lack of skills, experience, confidence and the attitudes of some employers can all wreak havoc on a disabled person’s likelihood of getting a job.
"The government has a responsibility to ensure taxpayers get a fair deal from the contracts it issues but it has an equal responsibility to ensure its departments and any contractors it works with treat disabled people fairly and with a duty of care.
"This is a flawed assessment that doesn’t take into consideration all the barriers disabled people face finding work.
"Crucially these high numbers of appeals show the government is not learning from its mistakes despite assessing up to ten thousand people every week.
"It is the government that has designed this deeply flawed assessment and until it makes fundamental changes to the design of this test, it will continue to have a devastating effect on disabled people’s lives."
Labour MP Tom Greatrex said the value for money of the £112m a year with Atos has been called into question by the NAO report.
"Through the Tribunals Service, the appeals are costing about £60m a year so effectively we are paying twice to try to correct the mistakes in the initial assessments or the process that leads to the assessments and decisions," he said.
Work and Pensions Secretary Iain Duncan Smith said the Work Capability Assessment has been "substantially improved" since the coalition took office in 2010.
"It is a complicated area but we are committed to making it a success to ensure it is both fair and accurate for the user and value for money for the taxpayer," he added.