"A further update will be provided in due course.
"Train timetables are presently being worked on and will be published as soon as possible.
"While local services are running, we expect them to be very busy. Rail replacement services are extremely limited.
"Customers are advised not to travel on Sunday, May 9 and refunds will be provided."
Routes that are affected include:
London Paddington - Swindon - Bath Spa - Bristol Temple Meads
London Paddington - Swindon - Bristol Parkway - Newport - Cardiff Central - Swansea
London Paddington - Reading - Taunton - Exeter St Davids - Plymouth - Penzance