Hundreds of staff at the DVLA in Swansea are set to begin a three-day strike in a dispute over pay.
About 650 members of the Public and Commercial Services union will walk out from Friday in protest at cuts to Saturday pay.
The union said staff recruited this year were not being paid an allowance for Saturday working, while others faced a 50% cut.
PCS says workers elsewhere in the civil service get extra weekend pay.
DVLA said in a statement that the strike could lead to a reduced service and longer waiting times.