Union leaders in Cumbria fear low-paid workers will suffer from a shake-up of the way council staff are paid.
Cumbria County Council said it would save nearly £120,000 a year by paying all its staff monthly instead of fortnightly or weekly.
It said it would save the money by having to administer only two pay dates a month instead of the current 14.
But Unison said it opposed the move, fearing low-paid workers would be left hard-up.
As a result of the changes, about 4,000 staff, including carers, cleaners and catering staff, will see changes in the way they are paid.
The council said the move would save £117,500 every year and will start at the end of August.
All affected staff are being written to and being given advice. An optional interest-free loan will be available through the council to help people with the transition period.
Councillor Liz Mallinson, cabinet member responsible for organisational development, said: "By making these changes to our payroll systems we'll be more efficient and more effective.
"It is a big piece of work involving 4,000 staff and it's important that we get it right.
"We're supporting our staff throughout the transition."
But Cumbria county branch secretary of Unison, Debbie Hamilton, said she was concerned about the impact it would have on some of their members.
She said: "We have huge concerns for those of our members who are low-paid and live week-to-week on their money.
"The pressure is going to be felt by those people at the bottom end of the pay scale."