Building the controversial Beauly to Denny transmission line would be an "entirely unnecessary act of vandalism", according to three senior industry and economic authorities who have complained that the public inquiry into the proposal will not let them explain why.

The three - Professor Andrew Bain, Sir Donald Miller and Mr Colin Gibson - have written an open letter to the press because of their frustration that the inquiry is not allowing crucial evidence to be aired which would expose what they claim are deep flaws in the proposals.

The letter's publication today comes after oil giant BP controversially shelved plans to build the world's first full-scale carbon capture power plant in Peterhead and ahead of the public inquiry into the Beauly/Denny line opening its first local session in Inverness on Tuesday.

Mr Bain, an economist and former board member of Scottish Enterprise; Sir Donald, who chaired both the South of Scotland Electricity Board and ScottishPower; and Mr Gibson, a chartered engineer and former Power Network Director of the National Grid Group, have all signed the letter condemning the proposal to build the 136-mile transmission line.

The 400,000 volt transmission line, which would go through the Cairngorms National Park, is deemed necessary to transmit all the extra power which is expected to be generated by renewable energy projects such as wind farms in the Highlands and Islands.

Under the £320m proposal submitted to the Scottish Executive by Scottish and Southern Energy (SSE) and ScottishPower, there will be 600 pylons, 200 fewer than the existing 132,000 volt line. But they will be much higher, up to 213ft high compared with the Scott Monument's 200ft and the Statue of Liberty at 152ft.

Mr Bain told The Herald last night that they had been prevented from challenging the economic and technical case presented by the developers.

He said the economic appraisal had been prepared by consultants and that "the assumptions in it are quite fanciful. It is a piece of advocacy rather than an independent report to my mind. My response should have been in by January 9, but I didn't actually see it until January 24".

He continued: "In addition, Sir Donald Miller prepared a very detailed report on the East Coast alternative route which would be cheaper and certainly less environmentally damaging, but he wasn't allowed to submit it. So it is not before the inquiry either.

"The applicants' case is that the Beauly/Denny line is technically necessary and economically justified. Neither of these arguments stands up in our view, but we were unable to present crucial counter-arguments. We don't believe that it is possible to ventilate the facts properly at the inquiry. That's why we are writing to the press."

It is not the first time the conduct of the inquiry has been criticised. A major legal submission made to the inquiry in Perth two months ago has still not elicited a response. This argued that no further evidence be heard and the application be refused.

Campaigners claimed a major flaw had been found in the applicants' case.