Rail workers on one of the busiest train lines from London to the South-west are to go on strike on the August Bank holiday.
Staff at First Great Western will launch two walkouts later this month in what would be expected to cause widespread disruption.
The first would be on Sunday 29 August and the second between Saturday 29 and Monday 31 August.
The RMT Union said that the introduction of the new fleet of Hitachi inter-city trains by FGW is causing concerns over safety, job protection and services.
The union says that the rail company has failed to "address core issues" of the Hitachi development plans.
RMT has instructed all members excluding fleet maintenance workers not to book any shifts.
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The action follows the 48-hour strike which took place last month leading to widespread disruption to services. The issues in dispute remain the same, RMT says.
"Despite strenuous efforts by our negotiators we have not been able to secure the kind of progress required in the key areas of jobs, services and safety for us to reach an agreement," a union spokersperson said.
"As a result we have no option but to put further action on."
