“Fit for work” tests are “significantly biased” against claimants in poor areas of the country, new research seen exclusively by the Guardian shows.

Analysis of more than a million incapacity benefit claimants who have been re-assessed for employment and support allowance (ESA), the benefit for people too disabled or ill to work, shows the controversial work capability assessment is disproportionately removing benefits from people in more deprived regions.

At the same time, it found claimants in wealthier areas are more likely not only to retain their sickness benefits – and avoid being declared fit for work – but to be placed in the support group of ESA, in which claimants are not required to undertake any form of work preparation and receive the highest benefit rate.

The study analyses government data from over 300 local authorities between 2007 and 2015 and is published next week in the journal Radical Statistics. It further shows that, counterintuitively, healthier areas found claimants fit for work less often, and placed claimants into the support group more frequently, than areas with higher rates of disability and illness.

“With an accurate test of disability, it would be expected that areas with lower disability and ill health would find more people fit for work,” Jonathan Hume, the author of the research, says. “As it stands, areas where people have the most need for ESA – so higher poverty, higher disability – are the ones removing it at a higher rate.”

Such findings are consistent with the idea that, as part of wider reduction in support for disabled people, the “fit for work” test was designed to cut support rather than better target it to those who “truly need it” as the government has repeatedly claimed, Hume adds.

There have been widespread concerns about the validity of this test in recent years, with critics saying it is inaccurate, discriminates against mentally ill people and those with fluctuating illnesses, and causes widespread stress and even suicidal feelings among claimants.

Ministers pressed ahead with the rapid expansion of the test to reassess 1.5 million existing incapacity benefit claimants in May 2011, despite warnings from their own adviser, Professor Malcolm Harrington, that the rollout should be delayed until the test could be improved.

The fact that this latest research analyses people previously on the benefit is significant because it shows that they were all assessed as meeting a minimum level of disability. As such, anyone subsequently found fit for work by a work capability assessment had been previously identified as having a significant health problem.

In order to determine the causes and extent of any bias in the test, it would be necessary to have access to data about individual claimants – for example, their disability, income and background – which is currently unfeasible due to confidentiality concerns. Still, the research has established a significant relationship between work capability assessment outcomes and local educational attainment. In areas where children finish school with more GCSEs, claimants were placed into the support group more frequently rather than being placed in the work-related activity group – the group in which disabled people must undertake preparation for a return for work or risk having their benefits sanctioned.

Hume says that a possible explanation of this is that people with more qualifications might be more able to complete the significant paperwork required to claim ESA, or are better at seeking appropriate evidence and assistance.

“Ultimately the DWP [Department for Work and Pensions] is making decisions that seem to be influenced by factors other than the health of the claimant,” he adds.

A Department for Work and Pensions spokeswoman criticised the report and said it makes a huge leap in attributing local variation in work capability assessment outcomes to “bias”. She said: “The report is entirely misleading and we don’t recognise the findings. The fact is eligibility for employment and support allowance is not dependent on the area where a claimant lives, but rather on the effect a person’s health condition or disability has on their ability to work.”

The Spartacus Network, a group of disabled campaigners who have previously undertaken influential research into ESA, says the study offers disturbing findings in an area that has so far been seriously overlooked. “The lack of previous analysis in this area is indicative of successive governments’ disregard for the effectiveness of ESA and the work capability assessment, compounded by indifference to the rights and concerns of sick and disabled people,” says the Spartacus researcher Caroline Richardson.

The findings are particularly relevant in light of the government’s impending cut of £30 a week for many new ESA claimants, due to take effect next year.

With the UN investigation into the UK’s alleged violations of disabled people’s rights due to be published in 2017, all eyes are on whether fit for work tests – and the wider disability benefit system – are themselves fit for purpose.