Services from London Paddington to south Wales and the West Country as well as routes in Oxfordshire and Berkshire could face disruption tomorrow after talks to avert a strike by the Rail, Maritime and Transport (RMT) union failed.
Members of the RMT will stage a 24-hour walkout tomorrow, with another stoppage planned for the upcoming Bank Holiday weekend.
First Great Western, which operates the routes affected, said it expected to be able to run about 70 per cent of its regular Sunday service and advised customers to check before travelling.
The row with the union is over the introduction of new trains. An FGW spokesman said: "The brand new Super Express Trains will deliver more seats, faster journey times and more frequent journeys for our passengers. Our plans mean more train managers and customer hosts on board our trains, not less."
But the union says the plans will lead to job losses and cuts to catering facilities on board.
Mick Cash from the RMT said: "Athough we have held many hours of talks with First Great Western we have not been able to make enough significant progress over the core issues at the heart of the dispute - drivers taking over the safety-critical doors and despatch functions which downgrades the safety-critical role of guards, removal of catering services and the threat to jobs among the train maintenance engineers.
"The action goes ahead and we have no doubt that it will be solidly supported this weekend."
He said the union was open to further talks next week.