From the FGW website :

Line problem in the Cardiff Central area.
Train services between Cardiff Central and Newport South Wales are being disrupted due to signalling problems in the Cardiff Central area.Engineers are working as fast as possible to restore services to normal. Short notice alterations and delays of up to 60 minutes can be expected.

05:27 London Paddington to Swansea due 08:56
This train will be terminated at Bristol Temple Meads.It will no longer call at: Filton Abbey Wood, Newport South Wales, Cardiff Central, Bridgend, Port Talbot Parkway, Neath and Swansea.This is due to signalling problems.

06:30 Cardiff Central to Portsmouth Harbour due 09:54
This train will be started from Bristol Temple Meads.It will additionally call at: Keynsham, Oldfield Park, Freshford and Avoncliff.It will no longer call at: Cardiff Central, Newport South Wales, Severn Tunnel Jn and Filton Abbey Wood.This is due to signalling problems.

06:59 Swansea to London Paddington due 09:59
This train has been cancelled.This is due to signalling problems.

07:30 Bristol Parkway to London Paddington due 09:00
This train has been revised.

This service will operate as an additional service from Bristol Parkway to London Paddington.

08:00 Cardiff Central to Taunton due 10:03
This train has been revised.It will no longer call at: Nailsea & Backwell, Yatton, Worle and Weston-Super-Mare.This is due to signalling problems.

09:30 Cardiff Central to Portsmouth Harbour due 12:52
This train will be started from Bristol Temple Meads.It will no longer call at: Cardiff Central, Newport South Wales and Filton Abbey Wood.This is due to an earlier train fault.

09:55 Cardiff Central to London Paddington due 12:01
This train will be started from Newport South Wales.It will no longer call at: Cardiff Central.This is due to signalling problems.

10:15 London Paddington to Cardiff Central due 12:22
This train has been cancelled.This is due to signalling problems.

10:22 Portsmouth Harbour to Cardiff Central due 13:41
This train has been started from Fareham.It will no longer call at: Portsmouth Harbour, Portsmouth & Southsea, Fratton and Cosham.This is due to earlier signalling problems.