Bus drivers and depot staff will take part in a 24-hour strike on Friday, which may cause disruption to hundreds of passengers.
First Devon and Cornwall announced yesterday that the company had been informed by union representatives that a 24-hour walk-out will be staged, affecting all First depots in both counties.
Transport union RMT – the National Union of Rail, Maritime and Transport Workers – say they are fighting to protect workers' standards of living after news following a pay dispute between workers and management.
Last week RMT members voted by almost 85% for strike action.
Bob Crow, RMT general secretary, said there were more than 100 First staff in Plymouth and the vast majority were union members.
The strike follows the rejection of what the union called a long overdue pay offer.
Chrissie Bainbridge, service delivery director for First South Coast, said: "We are incredibly disappointed that the Union has called for this action. We are now putting together contingency plans, which we will communicate to customers as soon as possible."