Derby City Council staff have voted in favour of industrial action over losing their car allowances.
Nearly 900 personnel currently receive more than £1,000 a year with more getting a special mileage rate.
The council told staff replacing this with a flat mileage rate of 40p would save hundreds of thousands of pounds a year and help reduce carbon emissions.
But Unison said the money was a vital part of the jobs package and members may refuse to use their cars for work.
A union spokesman added that if no progress was made with the council, action could begin as early as June.
The council has declined to comment.