The Department for Work and Pensions is to bring in additional providers alongside Atos Healthcare to administer the work capability assessment (WCA) for disabled benefits claimants, after a government review admitted that reports by Atos assessors were of unacceptably poor quality.

The announcement by the employment minister, Mark Hoban, follows months of criticisms of Atos which the government had so far either rejected or sought to address through fresh reviews of the scheme.

The announcement is likely to lead to new firms being brought in on a regional basis from summer 2014. Hoban said the extra firms will also help provide extra capacity to help tackle waiting times.

There have been long-term concerns that the system was unfairly weighted against people with health conditions that fluctuate, as the test assesses whether they could work on "the majority of days". There has also been criticism of the time it takes for appeals against decisions to be either upheld or rejected.

During interview, Atos assessors award claimants points reflecting the apparent severity of their condition and a computer program then calculates the score. Claimants who score 15 points are likely to be found eligible for support, while those with a lower score are not entitled to employment support allowance (ESA).

About 30% of those refused ESA support go to appeal and are subsequently granted the benefit. There have been more than 600,000 appeals since the WCA started, costing about £60m a year.

Hoban said he had already directed Atos Healthcare to put in place a quality improvement plan following a DWP audit which identified an unacceptable reduction in the quality of written reports produced following assessments.

Apparently drastic measures include retraining and re-evaluating all Atos Healthcare professionals, with those not meeting the required standard continuing to have all of their work audited until they do, or having their approval to carry out assessments withdrawn by the department.

The quality of the reports produced by Atos following an assessment are graded A-C, and the audit showed that the number of C-grade reports was around 41% between October 2012 and March 2013. A C-grade report does not mean the assessment was wrong, and the recommendation given in a C-grade report may well be correct but, for example, the reasoning behind that recommendation may lack the level of detail demanded by the DWP.

The DWP said this did not mean previous claims processed by Atos were wrong or subject to additional legal challenge. The reports provided by Atos form only a part of the WCA process, which has a number of checks and balances built in to the system, the department said, including an annual independent reviews of the WCA, from which over 50 recommendations have been, or are being, implemented.

The fourth independent review of the WCA is currently being undertaken by Dr Paul Litchfield. The DWP also said that a claimant whose report has not met its quality standard has been no more likely to be found fit for work, or to appeal against their decision, than other claimants.

Hoban said: "I am committed to ensuring the work capability assessment process is as fair and accurate as possible, with the right checks and balances to ensure the right decision is reached. Where our audits identify any drop in quality, we act decisively to ensure providers meet our exacting quality standards.

"Since 2010 we have made considerable improvements to the system we inherited from the previous government. However, it's vital we continue to improve the service to claimants, which is why we are introducing new providers to increase capacity."

The DWP has also engaged PricewaterhouseCoopers to provide independent advice in relation to strengthening quality assurance processes across all its health and disability assessments. Atos Healthcare has also brought in a third party to assess the quality of its audit and make recommendations for improvements.

Atos Healthcare said in a statement: "We continue to provide work capability assessments on behalf of the Department for Work and Pensions and continue to support the need to increase the number of health professionals on the ground to minimise waiting times and improve the system for those going through it.

"Our priority is the quality of our work and, following the recent audit, we quickly put in place a plan to improve the quality of written reports produced following an assessment.

"The professional and compassionate service we provide to claimants and the wellbeing of our people remain our primary consideration.

"We are sorry when we do not meet our own high standards but can reassure that a C-grade report does not mean the assessment was wrong and there are checks and balances throughout the system so that the correct decision on benefit is made by the department."

Mark Lever, the chief executive of the National Autistic Society, said: "It's unacceptable that Atos assessors' written reports which help decide whether people receive employment and support allowance or not aren't up to scratch.

"Sadly this revelation comes as no surprise. DWP and Atos reassurances that there are checks and balances to prevent wrong decisions being made seem pretty empty when recent figures show that the cost of the appeals has spiralled at a huge cost to the taxpayer."