A RAIL union has claimed journey times from Cardiff to London via Newport will be slower than in the 1970s despite huge investment.

The Rail, Maritime and Transport union said its research revealed that some services between London, Bristol and Cardiff will be slower from December 2018 than in the days of British Rail despite the introduction of new trains.

Great Western Railway defended its services and said the union was not comparing like with like, describing its facts as incorrect.

A statement said: "The fastest Bristol-London journey time, quoted by the RMT, in 1977 was one hour 25 minutes. Following electrification and the new trains, this will be one hour 19 minutes, that's six minutes faster than in 1977.

"In 1977, six trains ran every two hours between Cardiff-London and Bristol-London, compared to 12 trains every two hours following electrification and the introduction of new trains. That's double the number of services.

"The fastest Cardiff-London journey time, quoted by the RMT, in 1977 was one hour 45 minutes. This will be the same with the introduction of electrification and new trains. However while there was around one train per day that achieved one hour 45 minutes in 1977, after electrification and the introduction of new trains there will be at least three trains a day between the two cities that achieve that time, and the majority of others will offer competitive timings.

"These trains will call at more stations than they did in 1977.

"The fastest journey between Swansea-London in 1977 was two hours 43 minutes, compared to two hours 38 minutes after electrification and new trains."

A Great Western Railway spokesman said: "The comparisons made by the RMT are simply wrong. The electrification of the mainline and the introduction of the new Super Express Trains will deliver a much improved service compared to 1977 including faster journey times and more frequent services.

"Even today we run twice as many trains as British Rail did almost four decades ago, with many more station stops, allowing more journey choices for millions more customers. Following electrification, this will continue to improve with trains stopping at more stations and delivering more comfortable, more frequent services, to and from more destinations than was ever conceived possible four decades ago."

RMT general secretary Mick Cash said: "This report demonstrates once again that rail privatisation has been one of the biggest ever post war policy failures. Privatised rail services are not only more overcrowded and expensive they are also slower.

"And of course as well as being publicly owned, BR trains were publicly manufactured for far less cost in the UK, as opposed to the new trains which are manufactured in Japan."

The union is in dispute with the company over jobs and catering services on the new trains.