Almost 7,000 cancer patients could lose up to 94 a week under changes set out in the Welfare Reform Bill, which is currently going through parliament.
Ministers argue the bill - the biggest change to the welfare system for more than 60 years - will make the benefits and tax credits system "fairer and simpler".
But charities have opposed some of the changes, with Macmillan Cancer Support saying it could push cancer patients into poverty.
Under the plans, many cancer patients will only receive Employment and Support Allowance (ESA) for one year, worth 94 a week.
Its loss will leave patients without financial support at a time when they are not well enough to go back to work or face barriers to employment, according to the charity.
It claims the government's statistics show that one year is not long enough for many cancer patients - with 94 per cent of people claiming ESA still needing it after one year.
Under the plans, some people with cancer will be placed in the ESA work-related activity group (WRAG), which means they have to do work activities in order to claim benefit. Those in WRAG who based their claim on national insurance contributions will only be able to claim this benefit for 12 months before it is means tested.
Elspeth Atkinson, Macmillan's director for Scotland, said: "Many cancer patients will lose this crucial benefit simply because they have not recovered quickly enough.
"The majority want to return to work as it can represent a milestone in their recovery and a return to normality, in addition to the obvious financial benefits.
"This proposal in the Welfare Reform Bill will have a devastating impact."