Pylons would be up to 200 feet high if the scheme goes ahead
The go-ahead for a power line from Beauly to Denny is expected to be given by the Scottish government in the next few weeks, BBC Scotland understands.
But as more than 18,000 objections have been lodged it is thought likely conditions will be attached.
The upgraded power line, which would need 600 pylons, up to 200ft in height, would connect renewable power projects in the Highlands to the national grid.
Environmentalists and green energy advocates are divided on the issue.
The new £400m transmission line would run from Beauly near Inverness to Denny near Falkirk.
It would carry 400,000 volts (400kV), replacing the existing 132kV line, which runs along a similar route.
Supporters, including Labour and the Green party, say the scheme should be approved to help meet renewable energy commitments.
But concerns have been raised over the impact to landscape of the massive pylons and a lack of detail over alternative options.
The controversial plan has been the subject of a public inquiry.
The Scottish government has had that inquiry's report since February.
Labour's leader at Holyrood, Iain Gray MSP, said: "We want to see this upgrade go ahead as quickly as possible and it is disappointing that the SNP have taken far too long to make their mind up.
"Scotland needs a government that will take much bolder action in order to boost our renewable energy sector and tackle climate change emissions."
The Green party's Patrick Harvie told BBC Scotland's Politics Show: "It has taken a very long time to get this decision out, we still haven't heard it and I think that's regrettable. We need to be pressing on and getting the work done."
He said ministers could go to the forthcoming climate conference in Copenhagen and boast that the scheme was under way.
Patrick Harvie said the scheme should be boasted about in Copenhagen
But Dennis Canavan, former MSP and current president of Ramblers Association Scotland, told the programme: "If it does get the go-ahead then it will be an unacceptable act of vandalism.
"In Scotland we're very blessed, we've got some of the most scenic countryside in the world.
"It's a national asset, it's something that we should be looking after not just for ourselves but for future generations."
Liz Smith, Scottish Conservative MSP for Mid-Scotland and Fife, said: "There will be many people across our local communities who are extremely concerned about the news that the Scottish Government is going to push ahead with this project.
"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."
The Mountaineering Council of Scotland's chief officer David Gibson said that if approved the project would damage some of Scotland's most important mountain landscapes.
He added: "Until we hear the government statement, and the extent of the conditions placed upon any approval, we must assume the worst, and that means sacrificing iconic landscapes, rather than taking logical alternative route choices."
A Scottish government spokesman said: "Ministers will make a decision on the Beauly-Denny power line before the end of this year taking into account all relevant factors."