The Great Western Main Line from London to Bristol could be electrified – but it will cost £800,000 a mile.

The move is part of a package of transport infrastructure projects unveiled by the Government, which also includes the controversial expansion of Heathrow Airport.

Electric trains are quicker, lighter, quieter, cheaper to operate and maintain, and emit less carbon dioxide than their diesel counterparts.

Network Rail has calculated that it would cost £800,000 per mile of track to convert from diesel to electricity.

The 118.5-mile line from Paddington to Bristol has between two and four tracks for its entire length, which means that the cost of the project would be between £275 million and £380m.

Railway operators and campaigners have welcomed the move. Network Rail, which is responsible for the UK's network of track, would be in charge of the project.

Chief executive Iain Coucher said: "Under Network Rail's stewardship, Britain's railways have more passengers than at any time since 1947, trains are running at record levels of punctuality and safety is at an all-time high.

"It is clear that this success creates challenges as more and more people want to travel by train. Notwithstanding our plans to invest almost £8 billion over the next five years adding capacity to the network, more is needed.

"It is therefore timely that the Government and Network Rail jointly examine the need for new high speed lines."

Ellie Banks, spokeswoman for passenger rail operator First Great Western, said: "We welcome electrification in the context of running the railway today and continuing to look after our customers.

"We would be happy to work with the Department for Transport and Network Rail on electrification proposals out of Paddington, on to the Great Western network.

"We are focused on running our current diesel trains until 2013, with a potential franchise extension until 2016. We currently run 125mph diesel trains and cannot envisage a fully electrified network in these timescales.

"There is evidence in the UK and around the world that suggests electrification is a good thing. Generally, electric trains are more reliable than diesel, providing the most reliable and punctual service in the UK."

Julie Boston, campaign coordinator for the Friends of Bristol Suburban Railways, said: "We believe this investment it is absolutely essential in the interest of efficiency and the environment.

"Most of Europe opted for electrification decades ago and our railway system has been left lagging far behind.

"Electrification will make the service more reliable and much more attractive for rails users.

"Rail travel in Britain is still the most expensive in Europe and anything which could help cheaper fares would be welcome."

Only 39 per cent of the country's rail network is currently electrified – one of the lowest figures in Europe.