The approval of the £350 million proposal will be officially announced in the “next two to three weeks”. Scottish and Southern Energy and ScottishPower applied for permission to upgrade the power line in September 2005. It prompted the biggest public inquiry since devolution and received more than 18,000 objections.

The inquiry, led by Reporter Timothy Brian, heard from almost 200 witnesses over 105 days and ended in February 2008. Its six-volume report has been sitting with ministers since February 18 this year.

The power line has polarised opinion. Supporters claim it is the most important piece of the jigsaw for meeting the Scottish Government’s 2020 environmental targets, which are among the most ambitious in the world.

However, a broad coalition of critics, ­including the National Trust for Scotland, five local authorities and the John Muir Trust, argue it is an unnecessary white elephant running 220km through some of Scotland’s most beautiful countryside, and spoiling views around the Wallace Monument and the Cairngorm National Park.

It will create the capacity to transmit around six gigawatts of power generated from wind, wave and tidal turbines in the Highlands and Islands to electricity users further south, comprising about three-quarters of the output needed to meet the country’s 2020 renewable energy targets.

It is understood that the inquiry report will be published around the same time as the announcement is made. One industry source said that the announcement would come “within the next two or three weeks”, which could tie in with the Scottish Renewables Marine Energy conference in Inverness on November 12, although other sources suggested ministers might want to time it along with the Connecting Europe conference in Edinburgh on November 24.

The approval of the power line has been cautiously welcomed by opposition politicians and industry figures. Patrick Harvie, co-convener of the Scottish Green Party, said: “Pretty much everyone in the industry knows that we can’t get the renewables onto the network unless we have the grid upgrade done. If they were to say no or make any more delays, they would be in serious danger of losing all credibility on renewables. We should be cracking on and getting the work done.”

Adam Bruce, chairman of the British Wind Energy Association, said the line was a “very necessary part of a larger upgrading of the grid infrastructure which we will need to achieve a transition to a low-carbon economy”.

Lewis Macdonald, energy spokesman for Scottish Labour, said that a positive decision would be “extremely welcome” but complained about the time that it had taken to come. “They have taken their time about it,” he said. “Clearly it was important that they made the right decision, but they set a precedent for quick decisions by deciding on the Trump application in 20 days. Taking months and months over something like this makes it look like they are not giving it the same priority. Golf courses are important but electricity lines are even more important.”

Gareth Williams of the Scottish Council for Development and Industry said he would “warmly welcome” the decision, were it officially confirmed.

“Businesses and consumers will see the Scottish Government as having made the right decision,” he said. “We need to decarbonise our electricity supply if we’re going to avoid economically and environmentally catastrophic climate change, and if the Scottish economy is to grow in the long term, we need to take advantage of our competitive advantage in renewable energy resources.

“The recent report from the Marine Strategy Group identified the Beauly-Denny line as the top priority if Scotland’s wave and tidal power is to be harnessed and transmitted through the grid.”

Longstanding opponents of the power line are enraged at the decision. Caroline Paterson, who led widespread opposition to the project on health grounds -- primarily citing a link between overhead power cables and childhood leukaemia -- described it as “a terrible decision”, adding that it imposed “a terrible blight on the health of the people of Scotland for generations to come”.

Helen McDade, of the John Muir Trust environmental group, said: “Contrary to popular spin, it’s not just an upgrade on a line that’s already there. The impact of the pylons that will be involved will be very significant.” She said the inquiry had failed to properly consider alternatives, such as either reinforcing the east coast grid line or connecting power from subsea cables around the British coastline. The latter option was dismissed as too expensive, even though the cost of building interconnectors between Stirlingshire and England to cope with larger power transmissions had not been factored into the figures, she said.

“Also, the inquiry took place with the absolutely adamant position from Scottish and Southern Energy [the power company that would build the upgrade] that they would not consider undergrounding. If the decision has some undergrounding in Stirling and Cairngorm National Park, the costs become hugely different. Yet the reason why the proposal won the approval of [regulator] Ofgem was on the low costs,” she said.

With the backing of groups including the National Trust for Scotland, Cairngorm National Park, the Ramblers Association and five Scottish councils, she said that John Muir still hadn’t ruled out taking legal action.

A spokesperson for the Scottish Government said: “Ministers will make a decision on the Beauly Denny power line before the end of this year taking into account all relevant factors.”