Plans to extend the M74 motorway through South Lanarkshire and Glasgow could be jeopardised by a ruling from the European Court of Justice (ECJ).
The ruling means further costs to clean up the route
Much of the land the £500m road is to be built on is contaminated by chromium dumps buried underground.
Europe has now ruled that all such land must be classified as hazardous.
That means millions of pounds may have to be spent on cleaning up the ground, adding to the cost of the M74 project, or even halting the scheme altogether.
The judgment, involving a case in Belgium, stated that contaminated soilshould be treated as waste and it could, according to environmentalists, have major consequences for how such land can be treated or developed.
Scotland has an estimated 3,580 hectares of contaminated land, including parts of the site for the planned six-lane elevated extension to the M74 through south-east Glasgow.
The Scottish Executive claims the completion of the M74 could boost the economy, create more than 12,000 jobs and cut congestion
It said that executive lawyers were still studying the ECJ ruling.
The M74 proposal was the subject of a public inquiry following nearly 400 objections.
Friends of the Earth (FoE) said that that last week's European ruling meant the executive's plan to seal over the contaminated land with concrete could be illegal.
FoE Scotland chief executive Duncan McLaren said: "It is time that this contaminated land was cleaned up, not just covered up.
"We've always maintained that the M74 project is environmentally and socially unsustainable.
"The failure to properly deal with the contaminated land on route is only one in a long list of failings by the Scottish Executive and the road's other proponents."
Campaigners, Joint Action against the M74 (JAM74), called for the motorway plans to be ditched and the public cash diverted to clean up contaminated land.
JAM74 chairman Will Jess said: "If the Scottish Executive had any interest in improving the health of Glasgow's people, it would spend the £1bn due to be wasted on another polluting motorway on cleaning up contaminated land across the whole of Glasgow.
"Concreting over toxic waste under the M74 is no solution to the wider issues of improving Glasgow's polluted environment."
The Scottish Executive's Environment and Rural Development Department refused to comment directly on the claims.
The motorway plans have caused controversy
A spokeswoman said: "Scottish Executive solicitors, along with those from the Department of Environment, Food and Rural Affairs, are looking at this ruling."
Patrick Harvie, Green MSP for Glasgow, said: "Just because we have to clean up the land, does not mean we should run away from it and damage more greenfield sites instead.
"If half a billion can be found to build this unnecessary road, then clearly money can be found to clean up pollution. This ruling could signal the start of a proper clean up."