A NEW high-speed rail line in the UK would produce, at best, only very modest environmental benefits, a report commissioned by the RAC Foundation said today.

Taxpayers would have to foot a bill of between £17 billion and £27 billion for the construction costs of the London to Scotland line, the report added.

Not enough work had yet been done to see whether the money for the line would be better spent on other areas of transport, the report also said.

A high-speed line would take people off cars and planes but could also generate a lot of new travel, the report concluded.

The report was written by Professor John Preston of Southampton University's transportation research group.

The case for a London to Scotland high-speed line is currently being considered by High Speed Two (HS2), a body set up by the Government.

Today's report by Prof Preston said many of the green credentials of high-speed rail (HSR) would depend on the eventual load factor (the ratio of passengers to seats).

It added that European HSR systems had shown very varied load factors.

Also, across a sample of five European HSR routes a quarter of all travel trips in the area covered by the new line were new.

The report said that the recent studies on north-south high-speed lines had been ''uni-modal'' and did not offer comparative examples of the benefits to be derived from investing in other transport projects, for example congestion-relieving road schemes Prof Preston said the predominant benefits of a north-south high-speed line would be time savings for rail users and net revenue to the rail industry.

There would also be benefits in terms of reduced congestion on existing conventional rail lines - for example the West Coast main line.

Commenting on the report, RAC Foundation director Professor Stephen Glaister said: ''Many politicians are almost evangelical in their support of HSR. But they must be careful not to let their zeal be dictated by uncritical enthusiasm rather than hard facts.

''All the main political parties have been tempted to back a north-south high speed rail line in the UK even before HS2 has reported back (to the Government). And they also seem to be happily ignoring the wealth of evidence already out there which contains wide differences of opinion on just how beneficial HSR would be to the travelling public, the taxpayer and the environment.''

Prof Glaister went on: ''In a world where every penny counts there are big questions to be asked about whether billions of pounds lavished on HSR would be money well spent, or whether it should instead be directed towards other types of transport project.''