Turn2Us said its research among benefit claimants found that 43% of those it spoke to were unaware of the timetable for the phasing in of Universal Credit, which will streamline a range of existing benefits with an initial national rollout from October, following several "pathfinder" pilot projects taking place from this month.
The charity said that three-quarters (77%) of those who are aware of the reforms say they do not know how or when this will start to affect their benefits.
Its research indicated that future claimants may have more problems managing their money, with 29% worried that the move to monthly payments will mean they may get into debt.
Alison Taylor, director of Turn2Us, which is part of Elizabeth Finn Care, said: "Our latest research findings have confirmed what we have been hearing from the people we help for a number of months; that there is real uncertainty, confusion and concern over the forthcoming changes, despite the fact that there has been much work by the Government to simplify the system.
Universal Credit is being phased in over a four-year period to eventually include eight million households, replacing a range of existing benefits such as housing benefit and child tax credits.
The reforms aim to simplify the existing system and make it more cost-effective to run as well as making it easier for people to return to work. More than 1,000 benefit claimants with annual incomes of £20,000 or less took part in the research last month.
A Department for Work and Pensions spokeswoman said: "Our welfare reforms will bring much needed fairness back to a benefits system that is so complex it's failing the very people it is supposed to help. We are carefully working with councils and housing associations as our welfare reforms are being introduced to ensure people that need help get it. We're contacting everyone affected to let them know about the changes and as they affect them."
People can also visit http://www.dwp.gov.uk/policy/welfare-reform/universal-credit/toolkit.shtml to find out more about the reforms online, she added.