The announcement follows plans confirmed late last year for electrification between London Paddington, Didcot, Newbury and Oxford.

Hammond said: “Extending electrification westwards to Bristol and Cardiff will also bring all the benefits of electric trains – faster acceleration, greater comfort and cleaner, greener travel – to rail passengers in Wales and the south west.

“We have also established that a strong high-level case may exist for electrifying some of the Valley lines north of Cardiff. My Department will now work with the Welsh Assembly Government to develop a business case for the electrification of the Cardiff Valley lines.”

Paul Plummer, Network Rail’s Director, Planning and Development, said: “These schemes will be delivered by adopting modern and efficient high-output methods, and we will work in partnership with our suppliers and train operators to keep costs down and deliver better value, while minimising disruption for rail users.”