Opponents have called for years for two other options to be considered by ministers, without success.

But almost unnoticed both have become Government policy, despite the official report of the public inquiry rejecting both the upgrading of the existing east coast route and laying a subsea cable as alternatives to Beauly-Denny on grounds of cost.

The report was given to ministers last February, and published on Wednesday.

But The Herald can reveal in the footnotes to Energy Minister Jim Mather’s announcement that the line would be given the go-ahead on Wednesday, there is confirmation that both these options will also be needed in addition to the 137-mile line.

They are included in the Scottish Government’s National Planning Framework 2 (NPF), which was published in June, but was overshadowed by announcements at the time about the new Forth crossing, high-speed rail link to London and Commonwealth Games facilities.

Mr Mather’s statement was accompanied by confirmation that the second national planning framework, approved by the Parliament, includes the upgrading of the east coast transmission route and the reinforcing the Beauly-Keith overhead transmission line.

The footnotes also confirm a commitment to pursue a subsea option, in partnership with the UK Government through the Electricity Networks Strategy Group (ENSG). It would mean “... developing off shore subsea cable links between the Scottish mainland and the islands and links to the UK for export”.

David Infield, professor of renewable energy technologies at Strathclyde University, said: “We have always regarded Beauly-Denny as an essential reinforcement. There is enormous potential for renewables and indeed in the longer term Beauly-Denny won’t be enough.

“I have just been looking at the consented but not built wind farms in the north of Scotland, and there is something like a thousand megawatts worth. That will gobble up much of the Beauly-Denny capacity. And that is without new wave or tidal stream power generation installations.

“We need to get on and get required transmission capacity installed now or developers will be discouraged from further investment.

“Beauly-Denny, alongside new east coast and sub-sea cables, will all be needed if Scotland is to truly exploit its considerable renewable energy potential.”

Opponent Helen McDade, of the John Muir Trust, said: “For ministers now to accept these other options is one of the most disingenuous parts of the whole process.

“We were told that neither the east coast line nor the subsea cable were suitable. Now we have the minister saying we need both and Beauly-Denny.”