From the FGW website :

Line problem in the Portsmouth Harbour area.
Train services between Fareham and Portsmouth Harbour are being disrupted due to signalling problems in the Portsmouth Harbour area.Engineers are working as fast as possible to restore services to normal. Short notice alterations and delays of up to 30 minutes can be expected.

Train services towards Portsmouth will terminate at Fareham and train services from Portsmouth will start at Fareham. Customers are advised to travel on South West trains services between Fareham and Portsmouth & Southsea. Road transport is provided between Portsmouth & Southsea and Portsmouth Harbour.

14:30 Cardiff Central to Portsmouth Harbour due 17:54
This train will be terminated at Fareham.It will no longer call at: Cosham, Fratton, Portsmouth & Southsea and Portsmouth Harbour.This is due to signalling problems.

15:22 Portsmouth Harbour to Cardiff Central due 18:45
This train will be started from Fareham.It will no longer call at: Portsmouth Harbour, Portsmouth & Southsea, Fratton and Cosham.This is due to signalling problems.

15:30 Cardiff Central to Portsmouth Harbour due 18:59
This train will be terminated at Fareham.It will no longer call at: Cosham, Fratton, Portsmouth & Southsea and Portsmouth Harbour.This is due to signalling problems.

16:22 Portsmouth Harbour to Cardiff Central due 19:45
This train will be started from Fareham.It will no longer call at: Portsmouth Harbour, Portsmouth & Southsea, Fratton and Cosham.This train will run short formed with 2 carriages.This is due to signalling problems.

16:30 Cardiff Central to Portsmouth Harbour due 19:54
This train will be terminated at Fareham.It will no longer call at: Cosham, Fratton, Portsmouth & Southsea and Portsmouth Harbour.This train will run short formed with 4 carriages between Bristol Temple Meads and Fareham.This is due to signalling problems.

17:22 Portsmouth Harbour to Bristol Temple Meads due 19:49
This train will be started from Portsmouth & Southsea.It will no longer call at: Portsmouth Harbour.This is due to signalling problems.

18:22 Portsmouth Harbour to Cardiff Central due 21:47
This train will be started from Fareham.It will no longer call at: Portsmouth Harbour, Portsmouth & Southsea, Fratton and Cosham.This is due to signalling problems.