Campaigners against the M74 extension in Glasgow claim the Scottish Executive is about to announce the go-ahead for the project.
The M74 extension plan has caused controversy
They said the cost of the road has now reached £1bn, more than twice the original estimate.
Opposition MSPs and environmentalists called a news conference at Holyrood to pledge a continuing campaign against the new motorway.
They claim it will devastate the poorest parts of Glasgow.
The scheme is intended to alleviate traffic congestion through the city.
It is expected the five-mile stretch of road will join the M8 near the city centre and if approved could be ready by 2008.
A campaign against the road has been mounted by JAM74 (Joint Action against the M74), a coalition of community, environmental and sustainable transport groups.
But protesters, including the Green Party and the Socialists said it will fail to tackle the traffic problems of the city.
However, a spokesman for the Scottish Executive said the extension would bring benefits locally.
He said:"We are committed to sustainable development and to developing a transport system which minimises impacts on the environment and improves health by building and investing in public transport.
"We now spend over two-thirds on buses, trains, ferries, cycling, walking and other forms of public transport.
"The M74 completion needs to be viewed in the context of this wider transport policy.
"At a local level the M74 is expected to benefit communities along the route by taking traffic off local roads with a knock-on improvement in air quality and a reduction in noise."