Mastercard, one of the world’s largest payment networks, has told financial institutions to stop allowing marijuana transactions on its debit cards, citing the federal illegality of cannabis in the United States.

The move affects cannabis merchants and customers in several states where marijuana is legal for medical or recreational purposes, but who face difficulties accessing banking services due to federal laws.

Mastercard said it became aware of the issue and instructed the financial institutions that offer payment services to cannabis merchants and connect them to Mastercard to terminate the activity.

“The federal government considers cannabis sales illegal, so these purchases are not allowed on our systems,” a Mastercard spokesperson said on Wednesday.