The National Trust has said the ideas proposed by the government to ease traffic congestion around Stonehenge are unacceptable, and claims that the ancient monument in Wiltshire is at risk of losing its status as a World Heritage site as a result.

The National Trust said it believes that “none of the options under the transport minister’s (Stephen Ladyman) consideration is worthy of this site and thus the threat to Stonehenge is now urgent, serious and imminent”.

Sir William Proby, chair of the National Trust, said in an open letter to Ladyman: “If the government is unable to commit to implementing an acceptable long-term solution for Stonehenge then it would be better to make no commitment at all.

“We should not tie the hands of future generations in our search for a solution to tackling traffic passing through this iconic landscape.”

The issue surrounds the large volume of traffic on the A303, which currently runs close to Stonehenge and the A344, which passes it on the northern side. Upgrading these roads would not affect the stones themselves, but the surrounding 2,200 hectares, which hold significant archaeological value.

The government proposal, after a round of consultation that began earlier this year, includes the following options: a 2.1km (1.3 mile) tunnel for the A303, a cut and cover tunnel (where a trench is excavated and roofed over), a northern route, a southern route and partial solutions involving substantial road engineering at the nearby Longbarrow and Countess roundabouts.

The National Trust remains insistent that “in order to preserve the integrity of the whole site – the stones and their setting – the absolute minimum requirement is a tunnel of not less than 2.9km (1.8 miles)”. The suggested 2.1km tunnel bisects an area of the land known as ‘the Avenue’, which is “a critically significant part of the ceremonial site.”

Not all of the government’s suggestions were opposed however. The National Trust conceded that introducing “modest traffic management measures at the Longbarrow roundabout” and “closing the A344 and grassing over the junction with the A303” could present short-term improvements.

Ladyman is expected to publish the results of the A303 Stonehenge Improvement Scheme Review in the next few weeks. Details: www.nationaltrust.org.uk