NETWORK Rail threw its weight behind building a new high speed rail connection between London and Scotland but conceded the £13bn price tag of extending the line north of Preston raised "considerable affordability issues".

The not-for-profit company laid out costed plans which would see 200mph trains run from London to Glasgow and Edinburgh, with a total journey time to either city of just over two hours.

It estimated the cost of the route at £34bn, saying that the full benefits of shifting passengers from road and air on to rail would only be realised by a route that went via Birmingham, Manchester, Warrington, Liverpool and Preston on to Glasgow and Edinburgh.

Iain Coucher, Network Rail's chief executive, said high-speed rail would "transform" Britain.

NR said such a scheme would generate almost £55bn of value over the next six decades, paying for itself 1.8 times over.

But the company conceded that an alternative £21bn plan to build a new line as far as Preston, with trains running north on existing track infrastructure, was far cheaper and represented almost as good value for money, though it would leave the total journey time from London to Glasgow and Edinburgh at around three and a half hours.

The study discounted a Tory proposal to build a new line to leave via Manchester, saying it was essential to plan a westerly route in order to ease congestion on the West Coast Main Line, which is due to reach full capacity by 2020.

It also raised questions over the benefits of building a connection to Heathrow, saying it would be mildly detrimental to the business case.

The announcement was greeted with a flurry of enthusiastic responses from environmentalists, business groups and politicians from across the political spectrum but also raised questions about how it would be funded, given the severe constraints on transport spending over the next decade.

Christian Wolmar, a leading transport commentator, said building a new route as far as Preston was a more "realistic" option.

Scottish Transport Minister Stewart Stevenson said "The Scottish Government is committed to ensuring Scotland's voice is heard in this work and we now look forward to working with the UK Government on the development of a UK-wide network of high speed services.

"It is important to stress that high-speed cross-border rail routes to both Glasgow and Edinburgh should connect with the existing network in Scotland to ensure all parts of the country benefit."

Brendan Dick, general manager of BT in Scotland, is spearheading a Scottish Chambers of Commerce campaign to ensure the high speed link includes Scotland.