This work means there will be severe disruption and substantially longer journey times

Elaine Holt, First Great Western

Planned engineering work is simultaneously being carried out between Swindon and Didcot.

Elaine Holt, First Great Western's director and general manager, said: "Our customers are going to be faced with extreme difficulties on Sunday.

"This work means there will be severe disruption and substantially longer journey times."

One train an hour will run from south Wales to Bristol and vice versa and there will be one train an hour from Bristol to Paddington via Swindon, Chippenham and Reading.

Rail travellers between Bristol and London faced similar delays in February after a speed limit was imposed on a stretch of track damaged by a landslip.

Relief services

Journey times will be "substantially longer" and there will be a shuttle train between Bath and Swindon, a special service from Bath to Bristol and buses between Swindon and Didcot.

Extra staff are being drafted into stations to help customers.

First Great Western has drawn up emergency timetables posted at stations, on its website, on train announcements and on customer information screens.