2.7 million people will struggle to cope with the new Universal Credit system, a report says.
The Payments Council says the move to monthly payments will lead to problems with budgeting income.
Benefits are currently paid every two weeks.
The government has made plans for some vulnerable people to keep their fortnightly payments, but millions more will fall outside this provision.
The Payments Council oversees card and bank transfers, and surveyed benefit claimants over the last month.