Planned reforms to the welfare system will leave cancer patients who do not "recover quickly enough" poorer, activists have said.
The Welfare Reform Bill – on its way through Parliament – would see about 7,000 cancer patients lose as much as £94 every week.
The changes, the most radical to affect welfare for over 60 years, are intended to make tax credits and benefits "fairer and simpler", ministers say.
However, Macmillan Cancer Support is warning it could leave patients in poverty, joining charities which oppose other changes.
The planned changes would mean most cancer patients would get the Employment and Support Allowance (ESA) of £94 a week for a year only.
The charity said that will mean they have no financial support if they are not well enough to work after a year or find it hard to overcome the barriers some cancer patients face in the workplace.
It cited government statistics as evidence, saying they show 94% of people on ESA need it for more than a year.
If the Welfare Reform Bill comes into force, some cancer patients will be among ESA claimants put in a work-related activity group. They will have to do some sort of activity to qualify for the benefit.
According to the Bill, people in that group whose claim is based on national insurance contributions will only qualify for 12 months, after which it becomes means-tested.
Experts from Macmillan Cancer Support said a patient whose partner works more than 24 hours a week or who earns £149 per week would lose all their ESA.
Chief Executive Ciaran Devane said: "We are urging the government to change their plans to reform key disability benefits to ensure cancer patients and their families are not pushed into poverty."