The disability benefits assessments system has "serious failings", with concerns about the quality and cost of the controversial tests, an influential committee of MPs has found.
Claimants were not receiving an "acceptable level of service" and the cost to the taxpayer of paying private firms to carry out the assessments was set to double to £579 million in 2016-17, the Public Accounts Committee said.
The Commons spending watchdog said that the Department for Work and Pensions (DWP) and its private contractors were making progress to reduce delays and improve the quality of assessments but the regime still fell short of expectations.
Subsidiaries of Maximus are responsible for the Employment and Support Allowance (ESA) and Fit for Work service, while Atos and Capita carry out Personal Independence Payment (PIP).
The committee said: "Claimants are still not receiving an acceptable level of service from contractors, with particular concerns for claimants with fluctuating and mental health conditions."
The average time for PIP contractors to return assessments to the DWP was an "acceptable" four weeks, while ESA assessments took 23 weeks on average.
The committee heard concerns from mental health charity Mind, Citizens Advice and the Disability Benefits Consortium about claimants' "poor" experiences.
"Citizens Advice noted that it had received some 30,000 inquiries about medical assessments in the last year and highlighted examples of unreasonable journey times - with individuals travelling up to 60 to 70 minutes - and double-booking of appointments," the report said.
The MPs also noted that contractors were not required to employ doctors to carry out health and disability assessments, instead relying on practitioners including nurses and occupational therapists.
Only Maximus - with 186 doctors within its workforce of 1,251 - recruits doctors to conduct assessments, the report said.
The MPs also warned that too many assessments failed to meet the required standard and called for a more effective regime for monitoring their quality.
Committee chairwoman Meg Hillier said: "The troubled history of this programme hammers home the importance of getting contracts right - and the importance of then holding contractors properly to account.
"In this case, poor performance has had a tangible human impact. We have seen some improvements but there is a long way to go before people being assessed can be confident of getting the service they deserve.
"Our committee heard evidence of the assessment process continuing to create anxiety for claimants; of double-booked appointments and arduous journey times. Some assessors simply do not understand particular medical conditions.
"Up to one in five reports sampled by contractors were below the required standard and there is also evidence that attempts to reduce delays have undermined the quality of assessments, many of which are subsequently overturned on appeal.
"These are serious failings that must be dealt with rigorously. We will expect to see evidence of a more enlightened approach to the needs of claimants, greater transparency over contractor performance and a renewed focus on improving the quality of assessments."
Phil Reynolds, co-chair of Disability Benefits Consortium, a coalition of over 60 different charities and policy adviser at charity Parkinson's, said the report "paints a worrying picture".
He said: "With around two out of three disputed benefit decisions being overturned on appeal, we're deeply concerned that the speed of processing claims is potentially coming at the expense of accurate and fair assessments. Prompt decision-making is important, but so is quality - people with disabilities or with a health condition should be able to expect both.
"There's no one-size-fits-all solution for assessing disabled people and we welcome the committee's recommendation for urgent and significant improvements to ensure that assessment providers and the Department for Work and Pensions have well-trained, knowledgeable assessors who are sensitive to the complex issues that people with disabilities and long-term conditions face and the needs that they have."
A DWP spokesman said: "As highlighted in this report, we have made good progress to improve health and disability assessments; greatly reducing the backlog and cutting waiting times.
"But we know there is more to do and remain committed to working with our providers to ensure claimants get the best possible level of service, and taxpayers the best value for money."