Engineering work is taking place between Skewen and Gowerton, closing some lines.
Rail replacement buses will operate between Swansea and Carmarthen.
The 08:04 Cardiff Central to Swansea service will depart Cardiff Central later at 08:26.
Check before you travel:
You can plan your journey using the National Rail Enquiries Journey Planner.
Replacement bus travel advice:
For helpful advice if you need to travel on a rail replacement service, including accessibility and bicycle information, please use our Rail Replacement Services page.
You can find the pick-up and drop-off location of rail replacement services by checking station signage or by searching for your station on our Find a Station page.
Please be advised that replacement vehicles may be busier than usual, and you should allow extra time for your journey.