Network Rail will electrify nearly 300 miles of Britain’s busiest railway track over the next decade after the government today gave its approval to a £1.1bn programme.

The plans, announced by Gordon Brown this morning, will transform the Great Western mainline, which runs from London to Oxford, Newbury and Cardiff, via Reading.

Electrification will reduce carbon dioxide emissions and will mean faster and more reliable services for millions of passengers.

The prime minister travelled on one of the routes to benefit from the scheme this morning, arriving at Paddington station in London to journey on the Great Western line to Cardiff for a cabinet meeting.

The Great Western route from London to Swansea is to be electrified over the next eight years at a cost of £1bn.

The government is also spending £100m on electrifying lines between Liverpool and Manchester, with the work taking four years.

At Paddington, Brown said: “This is the future. It is green, it is faster and it’s more reliable. This is about making the railways fit for the 21st century.”

Asked if the government could afford such a scheme now, Brown replied: “We have set aside money for this. It’s an important priority for us.”

Only about one third of the rail network is electrified at the moment, with the Great Western route the last of the major routes to be still predominantly using diesel trains.

The electrification will include the lines to Oxford and to Newbury in Berkshire and will also make possible the direct replacement of the ageing InterCity 125 fleet by electric Super Express trains.

Electrification will shorten the London to Swansea journey time - currently just over three hours - by about 20 minutes. The plans will involve installing hundreds of miles of electric cables as well as alterations to tunnels, bridges and stations on one of Britain’s oldest rail routes.

Travelling with the prime minister today was the transport secretary, Lord Adonis, who said: “We are electrifying 300 miles of track and we are also looking to extend electrification to other lines.

“There will be some disruptions while the work is going on but Network Rail plans to keep disruption to a minimum, with much of the work being done overnight.”

Lord Adonis went on: “Electrification will mean faster, quieter and more efficient trains, which break down far less often.”

Mark Hopwood, managing director of First Great Western, said: “We are really delighted with this news. It’s going to transform our route and provide cleaner and more environmentally friendly travel.”

The electrification announcement follows Network Rail’s consultation document on electrification earlier this year, which also made the case for electrifying the Midland mainline route.

Lord Adonis said today that the government did consider Midland mainline and would continue to consider it.