Universal credit claimants are losing out on hundreds of pounds a year if their payday falls too close to their assessment date, a charity has warned.
Being paid early as a result of weekends or bank holidays makes it look like a worker has earned more, as they receive two payments in a calendar month.
The figures skew how much they are then assessed as being entitled to in benefits, according to the Child Poverty Action Group (CPAG). It said one in 20 cases analysed by its early warning system, which uses evidence from welfare rights advisers to identify issues, was hit by problems with the monthly assessment system.
Alison Garnham, CPAG chief executive, said: "Universal credit isn't working for working people.
"Our early warning system shows claimants are often left flummoxed by how much, or how little, universal credit they will receive from one month to the next. But we believe most of the problems created by the monthly assessment system can be fixed relatively easily if the political will is there.
"The mass migration of families on to universal credit should not begin until these fundamental problems are resolved. In the worst cases, people are losing out on significant amounts of money, hundreds of pounds over the course of a year, simply because of when their paydays and assessment periods fall."
A Department for Work & Pensions spokesman said: "We are listening to stakeholders' concerns and working on issues regarding payment cycles and we will consider this report carefully."