UNESCO, which promotes the preservation of historical sites, says it may even decide to put the landmark on its list of world heritage sites in danger if it is not satisfied by changes to the proposal.

Transport Secretary Mark Harper granted a Development Consent Order (DCO) for the project on the A303 in Wiltshire in July.

A DCO previously issued for the £1.7bn National Highways scheme was quashed by the High Court in July 2021 amid concerns over the environmental impact on the UNESCO World Heritage Site.

The plans involve overhauling eight miles of the A303 to transform a stretch of road - which has only one lane in both directions - well-known for its traffic jams.

The proposals include digging a two-mile tunnel, which would remove the A303 from sight next to Stonehenge. As it stands, it goes directly past the world-famous heritage site.

However, the UNESCO World Heritage Committee has warned changes must be made to the road tunnel proposal before any work goes ahead.

"Over the years, the Committee and the Advisory Bodies have been clear and consistent that the proposed A303 improvement scheme should not proceed in its current form," it said.

It further warned the "western portal" of the tunnel and associated dual carriageway would have "significant and inappropriate adverse impacts on the physical and visual integrity of the property".

UNESCO has urged the government to move the western portal of the planned tunnel "as far west as reasonably practical" to minimise archaeological removals, as part of its recommendations.

Its committee has asked for a report from the government on the state of conservation at the site by 1 February next year.

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'In danger now'

In August, the Save Stonehenge World Heritage Site campaign launched a legal battle against the government's decision to press ahead with the plans, having successfully blocked the scheme in 2021.

The Stonehenge Alliance, the group behind the campaign against the tunnel plans, has garnered more than 225,000 signatures from people across 147 countries for a petition opposing the project.

Historian and president of the group, Tom Holland, said a further legal challenge was necessary to "halt a development that, if allowed to go ahead, will permanently and irreversibly desecrate the Stonehenge landscape".

The Stonehenge Alliance added: "Stonehenge World Heritage Site is in danger now, with the legal action challenging the [government's] decision the only thing keeping the bulldozers off the site.

"We can only hope that it succeeds, because placing the site on the List of World Heritage in Danger in 2024 could be too late."

A spokesperson for the Department for Transport told Sky News the project will benefit the site.

"The decision letter sets out in full the reasons why it has been concluded that on balance the benefits of the scheme outweigh the harm," they added.

National Highways' project manager for the scheme, David Bullock, said its objective is to "conserve and enhance" the site.

The project team is working closely with archaeology contractors, he said, who have "an extensive track record of work in connection with the Stonehenge landscape".

"We remain confident this scheme is the best solution for tackling a long-standing traffic bottleneck, improving journeys, bringing much-needed relief to local communities, and boosting the economy in the southwest, while returning the Stonehenge landscape to something like its original setting," he added.