Campaigners against an extension to the M74 have vowed to explore "every legal avenue" to stop it being built.
Transport Minister Nicol Stephen gave the go-ahead to the five-mile stretch of motorway, in Glasgow, overruling the recommendation of a public inquiry.
Business leaders welcomed the decision to build "the new vital artery".
However, pressure group Friends of the Earth said it was "probably the worst environmental decision ever taken by the Scottish Executive".
The M74, the main link between the west of Scotland and England, comes to an abrupt end in Cambuslang, five miles east of Glasgow.
The new link would extend the motorway to connect it to the M8, close to the Kingston Bridge, south of the river Clyde.
The project cost has been estimated at between £375m and £500m.
Mr Stephen said the public inquiry Reporter had not given "enough weight" to the positive aspects of the scheme as presented in evidence.
He said: "This project is a key element in completing the central Scotland motorway network.
The M74 extension plan has caused controversy
"We believe that the benefits of this project outweigh the disadvantages and that it's in the public interest to proceed."
Mr Stephen said it will reduce congestion and help create jobs.
The RAC Foundation welcomed the news as "a victory for common sense".
Sue Nicholson, head of campaigns for the RAC Foundation, said: "It will reduce congestion on the M8 and improve journey times."
Alan Wilson, chief executive of the Scottish Council for Development and Industry, said: "The road to nowhere will now become a route to success."
Liz Cameron, director of the Scottish Chambers of Commerce, and CBI Scotland director Iain McMillan also welcomed the decision.
Mr McMillan said: "The need for this new vital artery is beyond doubt and we fully support the Scottish Executive's decision to overrule the reporter's judgement."
Scottish National Party transport spokesman Fergus Ewing MSP said the extension was essential for Glasgow and the wider Scottish economy.
He said: "What we are most concerned about is that the executive had a consultation and ignored it, making this exercise a costly waste of everyone's time."
Scottish Green transport spokesman Chris Ballance MSP said: "We now have evidence that the M74 extension will be a complete waste of taxpayers' money and will not do what ministers claim it will achieve.
"Yet still the executive intend to steam roller ahead with it. Today is a bad day for environmental justice."
Mr Ballance dismissed claims of a jobs boon as "absolute fantasy" and assertions of congestion cutting as "backward thinking".
Scottish Socialist Party MSP Rosie Kane was "furious" at the announcement.
She said: "This is an absolute outrage.
"The Scottish Executive has driven a coach and horses through the whole concept of independent inquiries.
"The Reporter has recommended that the M74 should not proceed and yet the Scottish Executive has treated his report with contempt."
Friends of the Earth chief executive Duncan McLaren, said: "Those who argued for sustainable alternatives have won the arguments and been stabbed in the back by Scottish ministers.
"We will now be exploring every legal avenue, including the use of Judicial Review, to halt this motorway from being built."