The research shows almost four in 10 people (37%) who return to work after cancer treatment say they experience some kind of discrimination from their employer or colleagues, compared to just under a quarter (23%) in 2010.
The YouGov survey of more than 2,000 UK adults living with cancer who returned to work after treatment found one in 10 felt harassed to the point they could not stay in their job.
It also showed one in eight said their employer failed to make reasonable changes to enable them to do their job.
Macmillan claim patients have reported being 'denied time off for medical appointments', 'passed over by promotion' or 'feeling abused by their employer or colleague'.
Ciaran Devane, chief executive at Macmillan cancer support, said: "Employers are risking prosecution by flouting their legal responsibility to protect people living with cancer from unfair treatment and stigma at work.
"There needs to be far more understanding of cancer and how the effects of treatment may impact on people returning to work."
He added: "Going back to work after treatment can be very isolating especially if someone has been off for a while and has lost confidence or contact with colleagues."
The charity is calling for employers to fulfil their obligations to people returning to work after cancer treatment under the Equality Act 2010. It recommends making reasonable changes to their work environment or hours and ensuring they have a back to work plan.
