The Scottish Executive has defended spending £41m on land along the route of the planned M74 extension before it gave the project the go-ahead.
The M74 extension plan has caused controversy
The executive has decided to press ahead with the project despite a public inquiry ruling against it.
The land purchase aimed to minimise disruption to businesses forced to relocate because of the extension.
However, protesters said the move showed that public consultation on the plan had been a sham.
The five-mile extension will stretch the existing route from where it currently ends in the south east of Glasgow into the city centre via a new elevated six-lane motorway.
The advance spending came to light in a report in The Herald newspaper on Monday, which revealed that £5m had been spent before the public inquiry started and £20m before its findings were made public.
The inquiry concluded that the plan should be abandoned as it would generate traffic, endanger jobs and provide only short-term benefits.
Environmentalists and local people have long opposed the plans and have warned of direct action against the scheme.
FoE Scotland chief executive Duncan McLaren told BBC Radio's Good Morning Scotland programme: "We participated in this inquiry in good faith and the fact that the executive spent almost £20m before it got the report suggests that it was just a box-ticking exercise.
"I think morally, the executive has wasted the money of our supporters and the many local people who had put their money where their mouth was to object to this scheme."
Barry Shelby, of the Joint Action Against the M74 (Jam 74) protest group, said: "We had always suspected that the inquiry was going to be taken seriously by the executive and this is an indication of how everything was moving ahead behind the scenes.
The M74 extension will stretch five miles across south east Glasgow
"It's disappointing to discover theses decisions were taken in advance, when people thought they were taking part in a democratic process."
The Scottish Socialist Party also joined in their calls and claimed the news gave a "green light for direct action" against the construction.
Glasgow MSP Rosie Kane said: "In their arrogance, the Scottish Executive has given campaigners the green light to go ahead with direct action against the contractors on the M74.
"If the Scottish Executive has no regard for the law and due process, why should we?" she asked.
However, the executive said the land had cost less than 10% of the projected expenditure on the scheme.
A spokeswoman said: "When the M74 scheme was announced it was recognised that some businesses required a long lead-in time to allow them to relocate.
"To date around 16 properties or parcels of land have been acquired at a cost of £41m.
"It is worthwhile spending this money in advance of a final decision to proceed to ensure minimum disruption to these businesses and to secure jobs."
The executive believes the extension will create 12,000 jobs, reduce congestion and cut road traffic injuries by up to 50 a year.
The new road will follow a route from the Fullarton Road Junction near Carmyle to the M8 just west of the Kingston Bridge.
There will be junctions at Fullarton Road, Cambuslang Road, Polmadie Road and at Kingston.
It is estimated that the motorway could be built by 2008 and cost between £375m and £500m.