First Great Western (FGW) has issued details of disruption to its services if planned strikes by workers go ahead in the coming weeks.
It says 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.
FGW says services including those between South Wales and London, Plymouth and London, Cardiff and Portsmouth, Bristol and Taunton and Bristol and Gloucester will be affected.
The RMT Union says it disputes the level of services the company could run on strike days.
Transport Secretary Patrick McLoughlin has told MPs that the electrification of the Great Western main line will be prioritised over other projects in the midlands and north of England as part of a crackdown on delays and rising costs at Network Rail. The state-owned company which controls the railway tracks is due to electrify the line from London to Swansea.
The minister told Plaid Cymru MP Jonathan Edwards that the government remained committed to electrifying through to Swansea on schedule, though he couldn't confirm that meant by 2018. He did not mention the electrification of the Valley Lines, which will largely be funded through the Welsh Government.
There's disruption on First Great Western services between South Wales and London Paddington because of a signalling problem at Slough.
London Underground are accepting tickets for reasonable routes.
Tickets being accepted on via reasonable routes on other National Rail operator services.
Passengers are advised not to attempt to travel via this route. Disruption is likely to continue until around 10:00am.
All services between Neath and Port Talbot Parkway are suspended due to a line-side fire.
First Great Western, which runs services between South Wales and London, is promising improvements after it was given a 23-month extension to its franchise.
The UK Government says it'll carry on running the Great Western main line until September 2015 before bidding starts for a new long-term franchise.
Greater wifi coverage and additional sleep carriages are among the improvements announced.
There are delays of up to 40 minutes on First Great Western between Severn Tunnel Junction and Newport because of a signalling problem at Severn Tunnel Junction.
Repetitive on board announcements on trains in the Thames Valley will be scrapped after years of passenger's complaints.
First Great Western are retraining one thousand staff to keep their messages to the point after the company's research showed their excessive guard announcements were causing customers to switch off.
First Great Western have decided to reduced the number of announcements made on train journeys, following pressure from passengers. Research carried out by the train provider found, half of passengers don't listen to them.
A spokesperson for First Great Western says the company is 'extremely disappointed' that union workers are planning industrial action later this morning.
The planned industrial action follows a dispute over a colleague's dismissal from the company.