Scottish and Southern Energy and ScottishPower applied for planning permission to upgrade the line in 2005, prompting the longest public inquiry since devolution began, which received 18,000 objections.

Supporters say the upgrade is essential to enable power generated by future natural tidal, wave and wind projects in the Highlands and Islands to be transmitted to the central belt, ensuring the government meets challenging environmental targets by 2020.

Anti-pylon campaigners are furious that a decision had been leaked, two or three weeks before the government is expected to make its formal announcement.

A government spokesman confirmed: “Ministers will make a decision on the Beauly-Denny power line before the end of the year, taking into account all relevant factors.” He also said that the government would be making no further comment.

Holyrood silence had  opponents demanding that the government reveal the details behind the upgrade,especially whether parts of the line would be buried underground or relocated away from homes.

David MacLehose, chairman of Scotland Before Pylons, said: “It’s so important that ministers have the guts to stand up and make a formal announcement about an issue that will affect Scotland for generations.

“Scotland Before Pylons, advised by industry experts, still believes the need for the proposed Beauly-Denny power line remains unproven.

“High-voltage overhead power lines are universally disliked because of the huge impact on landscape, health, heritage, tourism, economic development and communities. Why else did 18,000 people object to the government in 2005 when these proposals were announced and only 45 wrote in support?”

Highland Lib Dem MP (right) Danny Alexander said: “After many months dithering, it is typical of SNP government that they choose to leak a decision in the media rather than announce it openly. A clear decision is desperately needed by all those involved.

“Upgrading of the transmission infrastructure is desperately needed by the renewable energy industry, but to be publicly acceptable, the decision must at least include a commitment to substantial undergrounding in the most sensitive areas, especially in the Cairngorms National Park and around Beauly.”

Liz Smith, (left) Tory MSP for Mid-Scotland, said: “There is no doubt about the need for an upgrade to our electricity grid, but I have never been convinced that other alternatives, especially upgrading the east coast route or building undersea cables, were properly examined.”

It is believed that the  full announcement could be made to coincide with the Scottish Renewables Marine Energy conference in Inverness on November 12 or alternatively at the Connecting Europe conference in Edinburgh on November 24.