Hundreds of guards at rail operator First Great Western will stage a 48-hour strike from 20 January.
Passengers have complained about FGW's services
The Rail Maritime and Transport (RMT) union said relations with the company had broken down over issues such as managers driving and guarding trains.
First Great Western (FGW) said it was disappointed and that it would ask the union to "keep talking".
The company runs services from London's Paddington Station to the West Country, south-west of England and Wales.
The RMT also announced plans to ballot 1,500 station staff and other workers at FGW to see if they also wanted to take industrial action.
"We are very open to finding a realistic solution to issues the RMT raises," a FGW spokesman said.
He said the company did use managers to crew trains on a Sunday as a "last resort" as FGW was a commercial operation working seven days a week.
"The RMT believe guards can choose not to attend on Sundays... the decision to keep services going because staff are unavailable is only taken when all other possibilities have been exhausted," he added.
"We will not compromise safety."
The general secretary of the RMT, Bob Crow, said: "RMT members have today made quite clear what they think of First Great Western's confrontational style of management.
"The votes reflect the anger that our members feel at the company's behaviour, and if it wants to avoid industrial action it should begin to deal sensibly with the range of issues involved and stop the routine use of managers to guard and drive trains."
FGW recently increased its unregulated fares by about 6%, saying its costs had gone up and it needed the money for investment.
But its service levels have been criticised recently, with watchdog London TravelWatch calling for the company to be stripped of its franchise.
Last year, about 2,000 passengers refused to pay their fares for a day between Bristol and Bath in protest at the state of services.
And complaints about a revamped timetable and overcrowding were also reported.
A passenger group was due to meet soon to discuss a possible boycott of services.