A rail company says services between Portsmouth and Cardiff will be hit if planned strikes by workers go ahead in the coming weeks.
First Great Western (FGW) said long distance and local services will be reduced on August 23 and over the bank holiday weekend.
Members of the Rail, Maritime and Transport union are due to take industrial action in a row over new trains, which they say threaten jobs and services.
First Great Western managing director Mark Hopwood said: “These brand new trains will deliver more seats, faster journey times and more frequent journeys for our passengers. Our plans mean more train managers on board our trains, not less.
“The key issue remains that we want the driver to close the doors - which is fundamental to our ability to run the faster, more frequent services customers should expect from a 21st century railway. This method has been used safely across the UK rail industry for decades.”
FGW said it expects to run most of its London Thames Valley trains, but services including those between South Wales and London, Plymouth and London, Cardiff and Portsmouth, Bristol and Taunton and Bristol and Gloucester will be affected.
No service will run between Weymouth and Westbury, while some services in Devon and Cornwall will be cancelled.
The RMT said it disputed the level of services the company could run on strike days.