The electrification of the Bristol to London railway line will only happen if Labour hang on to power.

Transport secretary Lord Adonis, speaking to the Bristol Evening Post at the Labour conference in Brighton, said work could begin in two years, following detailed "mile by mile" plans being drawn up.

The first electric trains could run on the route by 2017.

But the £1 billion plan, which would electrify the line between London and Cardiff, taking in Bristol Temple Meads and Bristol Parkway, could be scrapped under the Tories, the Cabinet Minister warned.

The opposition has indicated the scheme is too costly.

Lord Adonis said: "We have started now in the planning and I have given the go-ahead for the £1bn of investment.

"It will be two or three years before there is serious work on the ground because there is quite a lot of mile-by-mile planning to be done.

"It will go ahead as long as I'm Secretary of State and this government is in office.

"If the country were to vote for a change of government next year, I can't speak for the Conservatives. They haven't said if they support electrification or not. I can absolutely guarantee that if Labour stays in office, we will complete this electrification programme.

"I also think it is essential for Bristol that we do so. Bristol is one of England's major cities and that line should have been electrified a long time ago. It would be a tragedy for Bristol and the South West if the Conservatives were to cancel this project."

The benefits of making the switch from diesel to electric include shaving 12 minutes off the journey time from Bristol to London, improved reliability and less pollution.

He added: "It will mean a much more reliable train service, a far less polluting train service and some acceleration in train times.

"Electric trains are much simpler to operate than diesel trains, they have far fewer moving parts so the interval between breakdowns is longer."

Earlier this week, Lord Adonis announced a £14 million package to transform cycling facilities at railway stations. He praised Bristol for leading the way by becoming the country's first "cycling city".

He laid problems with buses in the city firmly at the door of the councils in the former Avon area. They, he insisted, had the power to offer greater subsidies to help cut fares and create bus routes to speed up journeys.