CAS said some families were losing out on £90 a week after signing on to the new benefit.
A growing number of jobseekers are being left out of pocket after signing up for Universal Credit, according to Citizens Advice Scotland.
The advice network said many people who moved onto the new benefit were seeing their income drop by as much as £90 a week, leaving them struggling to make ends meet.
Families with disabled children, single parents and those who previously received tax credits were said to be especially at risk of losing out under the new system.
The warning comes as a Westminster committee revealed jobcentre staff are failing to inform claimants that voluntarily moving to Universal Credit will leave them worse off.
MPs from the work and pensions committee accused the Department for Work and Pensions (DWP) of failing to give claimants enough information about how much they will receive under the new benefit and failing to protect those who suffer a major drop in income.
The DWP was also blasted for refusing to allow those who had moved on to Universal Credit to move back to their previous benefit – a lock known as the “lobster pot”.
Frank Field, committee chair, said: “In the history of humankind, has there ever been an example of a government introducing a fundamental welfare reform and none of its employees being able to tell if it will leave people better or worse off?”
Tory ministers had pledged that no claimant would be worse off under Universal Credit. However, the committee estimated that 3 million working families would lose out under the system, compared with only 2.4 million who would gain.
CAS is now urging the UK Government to ensure that anyone left out-of-pocket under the new benefit is compensated for their loss.
The organisation’s social affairs spokesperson Rob Gowans said: “Scotland’s CAB network is seeing a growing number of people being moved onto Universal Credit who are struggling to cope with a sudden drop in income.
“Families with disabled children, single parents and people who previously received tax credits can be up to £90 per week worse off on the new benefit.
“CAS would urge the UK Government to make additional transitional payments to people being moved to Universal Credit now, to make sure no-one is worse off as a result of the change.”
