But a route connecting London to Glasgow would only begin to represent a net reduction of carbon dioxide emissions over the next 60 years if the market share travelling by rail was increased from 15% to 62%, according to the study.

By contrast, building a new high-speed line between London and Manchester would not offer a carbon saving over the ‘business as usual’ scenario, once the emissions produced when constructing the route are taken into account, even if there was a complete shift from aviation to rail.

The study by consultants Booz Allen Hamilton, commissioned by the Department for Transport, was completed in 2007 but only recently published by the DfT.

It raises fresh questions about the environmental justification for building a new high-speed route, whose cost has been estimated at £29bn if it extended all the way to Glasgow.

But it did not look at the potential for car users to switch to the train or the emissions produced by people driving to airports, which are substantial.

Business groups in Scotland, who are lobbying for a line to extend north of the border, said the report strengthened their case.

Garry Clark, head of policy and public affairs at the Scottish Chambers of Commerce, said: “The only comparable high-speed route is Paris to Marseille, where the proportion of journeys made by rail increased from 22% to 65%.

“I certainly think it would be possible to achieve at least a 62% rail share of the market between Glasgow and London.”

Transport Secretary Lord Adonis has commissioned a company, High Speed Two, to present plans for building a high-speed connection between London and the West Midlands by the end of the year and to consider options for extending the line to Scotland in the future.

The Booz Allen Hamilton study examined the emissions likely to be produced by building and operating a high-speed line over the next six decades.

A spokesman for the DfT said: “High Speed Two is due to report at the end of the year and will produce a full assessment of the environmental impact of the proposed scheme.”

However, Stephen Glaister, director of the RAC Foundation, said: “It is unwise for the government to have committed so heavily to high-speed rail in advance of the completion of the High Speed Two review, when they already have available their own ­comprehensive studies calling into question the environmental benefits and suggesting much higher costs for the taxpayer.”

‘I think it’s possible to achieve a 62% rail share of the Glasgow to London market’