Construction is set to start next year on a controversial 3.3 km-long tunnel to replace a heavily-used highway next to the U.K.’s prehistoric Stonehenge circle in England unless a final legal challenge scuttles the $2.1-billion project.

In early November, U.K. transportation secretary Grant Shapps approved a proposal by Highways England for the 13-km highway that includes twin bored tunnels to replace a busy stretch of the A303 highway, which passes about 165 m, or 540 ft, from the monoliths.

The English Heritage, which manages the monument, and the National Trust, owner of the surrounding land, welcomed Shapps’ decision. “The long-term benefits to the public, history and wildlife will be significant – reuniting this remarkable landscape, reducing the sight and sound of traffic,” they claimed in a joint statement with the U.K.’s Historic England.

But critics say the new highway’s construction, including a new viaduct, would “trash” prehistoric artifacts distributed around the 25-sq-km World Heritage Site, according to Kate Fielden, a member of the archaeological protection group Stonehenge Alliance in the lead up to an earlier inquiry into the plan. “The integrity of the while site would be damaged,” she says.

In fact, Shapps’ decision overrules recommendations to reject the plan from the government’s own Planning Inspectorate and triggered a final assault by objectors.

“Five independent planning inspectors, having reviewed 1,493 documents and three weeks of oral evidence, concluded that the…proposals… would cause ‘substantial harm’ to the world heritage site and should be refused,” noted officials from the independent Council for British Archaeology. English Heritage and the National Trust have conflicts because they have “special ownership, management and commercial interests” in the site, they added.

Stirred by Shapps’ decision, the organization Save Stonehenge World Heritage Site is looking to raise up to $67,000 to challenge the decision in the courts.

Meanwhile, Highways England awarded an $11.3-million preliminary works contracts and commissioned $47 million of archaeological investigations, aiming to start work early next year.

Stonehenge is set in a landscape rich in monuments from the early Neolithic, late Neolithic and early Bronze Age eras, according to English Heritage.

Roughly 30.5 m diameter, the stone circle was erected about 4,500 years ago within an earthworks enclosure built some 500 years earlier. Of the 30 stones that would have formed the outer ring, 17 remain standing linked by 6 lintels. Another eight uprights and two lintels lie flat on the ground.

These stones are 2.4 m to 6.9 m tall and weigh generally 25 tonnes, according to English Heritage.

Numerous smaller “bluestone” monoliths, weighing 2 to 5 tonnes each are dotted within the outer ring.