A legal challenge has been launched against £1.5bn plans to dual parts of the A66 just as contractors were gearing up to start work.
The A66 Northern Trans-Pennine project, which received development consent last month, looks set to delay the project again.
National Highways said it would fully support the legal process as the challenge passes through court.
A spokesman said: “We have been designing and developing the A66 Northern Trans-Pennine project, and we will continue to carry out archaeological investigations and preparatory works including utilities diversions during 2024.
“At this stage it is too early to say how much of an impact the legal challenge will have on our programme and start of works.
“We are disappointed about the legal challenge as we believe our proposals will provide much-needed improved safety and reliability for drivers and businesses that use this vitally important route every day, while also delivering a boost to the regional economy.
Transport Action North has lodged the challenge against the granting of a development consent order claiming it is deeply flawed.
Chris Todd, TAN’s founder and director, said: “The £1.5 billion A66 Northern Trans-Pennine represents very poor value for money and will cause great harm to important landscapes and wildlife.
Delivery partners Balfour Beatty, Kier and Keltbray had been eagerly waiting for the green light to start the project, which is to be delivered under an enterprise-wide collaborative working agreement.
Firms were understood to be promising attractive wage rates to assemble the workforce for the project.
The project will upgrade the remaining 18 miles of single-carriageway sections and key junctions on the route between M6 Junction 40 (Penrith) and the A1 at Scotch Corner.
