Labour has come out in support of high-speed rail lines, such as Birmingham to London, in a major policy U-turn.

The Government announced a new inquiry into the case for high speed rail, as Transport Secretary Geoff Hoon insisted: “New lines have great potential and it is important that we start now to plan for future growth.”

Ministers have never officially ruled out building high speed rail links, but the Government’s attitude has previously been hostile to the idea.

It follows the Conservative pledge made at the party conference in Birmingham that a Tory Government would build a new line with trains running up to 190mph, which could cut journey times from London to Birmingham to 45 minutes.

The Labour leadership is also struggling to cope with a rebellion over plans to build a third runway at Heathrow Airport. Opponents of the expansion plan, including a number of Labour MPs in London seats, have been arguing that a high speed rail service, costing an estimated £20 billion, could provide an alternative solution to growing demand for air travel.

By taking commuters from Birmingham, Manchester, Leeds and possibly Scotland to the Eurostar terminal in St Pancras, London, it could allow people to travel quickly to destinations on the Continent without needing to fly, MPs claim.

A House of Commons motion signed by 78 MPs, including around 50 Labour MPs, “regrets the fact that provisions to improve high speed rail lines from Heathrow to major cities have not been fully explored.”

There have been reports that some Cabinet Ministers, including Environment Secretary Hilary Benn, have been encouraging the Labour rebels.

Transport Secretary Geoff Hoon has now issued a Commons statement announcing a new inquiry, to be led by Transport Minister Lord Adonis.

It will examine ways of making better use of existing transport networks, such as expanding motorways or improving rail lines.

But it will also focus on “longer term solutions”, Mr Hoon said.

He told MPs: “This will include consideration of wholly new rail lines, including high speed rail. We are committed to developing a modern sustainable rail system that supports economic growth.”

The Government had previously said it was waiting for Network Rail to finish its own inquiry into the potential for new rail lines - which is not due to report until the summer of 2009.

A major review of Britain’s transport network in 2006, produced for the Government by former British Airways chief executive Rod Eddington, appeared to rule out high speed rail lines, concluding that they would be expensive while the benefits would be “relatively modest”.

In October last year, Transport Minister Jim Fitzpatrick told the House of Commons that the estimated cost of a new line had shot up to £30 billion, adding: “Presently there is no justification case for such expenditure.”

The Government then published a major transport policy paper, which listed high speed rail as an option for improving transport links between London, Birmingham and Manchester but only as one of a number of possibilities, including expanding the M6 motorway. The new statement suggests the Government believes high speed rail may be needed even if existing transport links are improved.