Environmental pressure groups are taking a council to the High Court over the controversial decision to build a major road through rural Dorset.
The relief road is set to relieve congestion on the A354 and A353
Campaigners against the Weymouth relief road say the route would cut through an Area of Outstanding Natural Beauty (AONB), which is protected by law.
The Campaign to Protect Rural England (CPRE) and Transport 2000 are asking for a judicial review of the decision.
Dorset County Council has pledged to fight the action.
Plans for the £84.5m scheme, drawn up to relieve congestion on the A354 and the A353, were approved in April and work is due to start early next year.
The groups claim that in approving the road, Dorset County Council had failed to consider strict regional planning policies restricting development in AONBs.
The policy regarding development in AONBs states: "The conservation and enhancement of their natural beauty, wildlife and cultural heritage will be given priority over other considerations in the determination of development proposals.
"Development will only be provided for where it would conserve and enhance the natural beauty, wildlife and cultural heritage of the Area of Outstanding Natural Beauty".
Stephen Joseph, executive director of Transport 2000, said: "The damage caused to Hardy's ridgeway is just not worth it."
Shaun Spiers, chief executive of CPRE, said: "We would be failing in our role to protect the countryside if we did not take this important legal action."
So far the county council has been unavailable to comment further on the claims.
In June, the government decided not to call a public inquiry into the scheme, despite the criticisms by campaigners.