A legal challenge could delay the construction of the new Aberdeen Bypass, according to Transport Scotland.
The agency responsible for the road made the claim about a Court of Session action by Road Sense.
Opponents to the scheme have raised concerns over costs and the environmental impact.
The Scottish government gave the bypass the go-ahead after a long-running public local inquiry in 2008.
William Walton, Chair of Road Sense, said: "We have been forced into taking this action after years of being sidelined, stonewalled, ignored and misinformed. Scottish Ministers’ decision to limited the scope and remit of the Public Local Inquiry into the AWPR made the inquiry nothing more than window dressing, and the concerns and arguments of the majority of the 10,000 objectors to the AWPR were never properly heard.
“The court process will be the first time that arguments about the means by which the route for the AWPR was identified, selected, appraised and approved will be heard.
"The court will be the first body to give a detached, dispassionate assessment of the legality of the AWPR as it is proposed. These things should have been heard at the Public Local Inquiry, but were placed outwith the scope and remit by Scottish Ministers.”
The road, estimated to cost £395m, aims to provide a fast link between towns to the north, west and south of Aberdeen.
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