Rail delays between Swansea and London due to repair work
Engineering work will mean rail journey times between Swansea and London will be extended by up to 45 minutes.
Great Western Railway said emergency track repairs on Sunday would hit services to Bristol as well.
Trains from south Wales to London will be diverted between Newport and Swindon and will not stop at Bristol Parkway.
Services between Cardiff and Portsmouth will start and end at Bristol Temple Meads with an hourly shuttle train between Cardiff and Bristol Parkway.
Trains from Newport and Swansea will run about 25-30 minutes earlier, apart from the last service.