Thousands of seriously ill cancer patients will be forced to take medical tests and face "back to work" interviews, despite assurances from ministers that they would not make it harder for the sick to get welfare, charities have warned.

Buried in a report to ministers by Prof Malcolm Harrington, the government adviser on testing welfare recipients, are proposals to force cancer patients who are undergoing intravenous chemotherapy to prove they are too ill to work.

At present, patients who are unable to work because of cancer and the side-effects of treatments are allowed to claim the highest rate of employment support allowance (ESA), worth up to £100 a week. More than 9,000 cancer patients were placed automatically on the welfare payment from October 2008 to June 2010. However, the expert report says this "automatic entitlement" has encouraged dependency on benefits, "encouraging wrong behaviours from employers and stigmatising cancer as something that can lead to unemployment or worklessness".

Instead, cancer patients on chemotherapy in hospitals will now have to prove that they are too sick to work, and take part in the controversial work capability assessment to determine whether someone is eligible for benefits. If cancer patients are found able to return to employment they may also be required to participate in work-related practice job interviews, as a condition of receiving their benefit.

Such assessments have been attacked by charities amid mounting evidence that people with serious illnesses are being judged fit for work when they are not.

Cancer experts and 30 cancer charities argue that patients undergoing "stressful" cancer treatment – and who have to leave work – should be automatically eligible for ESA. Ciarán Devane, chief executive of Macmillan Cancer Support, said: "Cancer patients in the middle of treatment are, in many cases, fighting for their lives.

"Yet the government is proposing to change the rules so all cancer patients will have to undergo a stressful assessment to prove they are unable to work.

"This shows a clear disregard and misunderstanding of what it's like to undergo punishing treatment. Patients who previously had peace of mind would face the stress and practical difficulties of getting assessed for work they are too poorly to do."

The new policy also reverses the government's own position on benefits that can be obtained by cancer patients. Iain Duncan Smith, the work and pensions secretary, earlier this year amended regulations concerning ESA so that even patients who were likely to receive chemotherapy within six months would be exempted from medical tests and face-to-face interviews and get benefits automatically.

This move allowed him to taunt Labour, claiming: "It was this government who exempted cancer patients on chemotherapy in hospitals; they were not exempted by the previous government. Our record on this is therefore quite good."

Labour called on the government to "listen very carefully" to its critics on the issue. The shadow work and pensions secretary, Liam Byrne, said: "This government is simply not thinking hard enough about the special circumstances of cancer sufferers in this country. People with obviously serious conditions don't need to be tested and retested – they should be allowed to get on with their treatment."

The government has already been forced on to the back foot over its changes to ESA, designed to save more than £1.2bn. In the summer, David Cameron refused to back down over plans to cut up to £94 a week from ESA, a move that could affect as many as 7,000 cancer patients. Devane said: "To make matters worse, the government is pressing ahead with proposed changes in the welfare reform bill that will make 7,000 cancer patients lose ESA after 12 months simply because they have not recovered quickly enough. We hope ministers will rethink these proposals.

"Cancer is the toughest fight many people will have to face; the government should not be making it tougher for them."

The ESA replaces a range of incapacity benefits. All fresh claimants now undergo a work capability assessment, and 1.5 million existing recipients will be reassessed using the new system from this month. The welfare reform bill introduces a one-year time limit on those people claiming ESA in the "work-related activity group", who are expected to move into work.

In an email, Harrington said: "Macmillan provided me with compelling evidence that different cancer treatments can have an equally – and varied – debilitating effect on individuals. However, I agree with the government that forcing people to a life on benefits when they want to work is wrong." The government said that patients would be allowed to prove they were too ill to work with "documentation", and would not always be asked to undergo a complete assessment.

A DWP spokesman said: "This must be about an individual's needs. Our proposals would ensure a person would only be asked to attend a face-to-face assessment where absolutely necessary."