The government has rewritten plans to change benefits for cancer patients following criticism from charities.
But Macmillan Cancer says the changes could have devastating consequences.
Previously, only those receiving non-oral chemotherapy were eligible for unconditional assistance, now a wider range of drug and radiation treatment will be included.
But the charity says more patients will now be made to undergo medical assessments before receiving benefits.
The government said it had made the amendments because it was clear some patients were "unfairly missing out on support".
It had hoped to introduce the amended plans from April, but having failed to secure Macmillan's support, was launching a consultation to obtain a wider range of views.
The dispute centres on the Employment and Support Allowance (ESA) system, which divides claimants into two categories.
Those deemed to have limited capacity for work are placed in the Support Group and are not required to undertake any work-related activity.
Those who are deemed able to perform "work-related activities" which might help them eventually return to work face means-testing after 12 months.
Under the government's original proposals, only those patients whose chemotherapy drugs were injected into a vein, the abdomen or the spinal canal would automatically be placed in the Support Group.
Those receiving chemotherapy drugs orally - perhaps taking tablets at home rather than visiting a hospital - or receiving radiation only would go into the work-related activity category.
The consultation document states: "This was based on a view that invasive chemotherapy is more debilitating in most circumstances than oral chemotherapy. However, this view is no longer supported by the evidence."
It goes on to say that evidence provided by Macmillan "supports the view that all forms of chemotherapy may cause substantial debilitation" and severe side-effects.
The rewritten plans presume that any individual undergoing any form of chemotherapy or site-specific radiotherapy should be in the Support Group, without them having to undergo a medical assessment.
It says that in "a small number of cases, where the evidence indicated that the debilitating effects might be limited" individuals may be invited for a medical assessment to judge their fitness to work.
But Macmillan said it was still not happy with the proposals.
As well as widening the eligibility, guaranteed entitlement to entry into the Support Group has been amended to a "presumption" based on "a paper assessment".
Macmillan said that was an erosion of the legal protection given to patients and it was unclear what the paper assessment would involve.
"We don't agree with this proposal and remain of the view that automatic entitlement is vitally important for cancer patients receiving treatment," the charity added.