Plans for 1.8-mile road tunnel are still very divisive and estimated cost of project has risen to £1.6bn

The government has changed the preferred route of a road tunnel designed to restore peace and tranquillity to Stonehenge after protests by druids, archaeologists and conservation experts.

There was an outcry that the 1.8-mile tunnel would pass too close to precious barrows and ruin the view of the sun setting at the winter solstice, a crucial date in the pagan calendar.

On Monday the government said it was now planning to shift the tunnel an extra 50 metres away from the prehistoric monument to try to ease the concerns of the scheme’s critics.

Opportunity or threat? Stonehenge tunnel plan revives long-running debate Read more

But the proposals, announced as part of plans for a £1.6bn upgrade to the route in and out of the south-west of England, are still very divisive, with sceptics arguing that precious ancient features will be destroyed and the site’s world heritage status could be at risk.

There is also concern that the cost seems to be rising. In January it was widely reported that the cost would be £1.4bn. The Department for Transport said the estimated cost had been revised upwards, to £1.6bn.

Chris Grayling, the transport secretary, said the investment in road improvements, including eight miles of dual carriageway as well as the tunnel, would be a huge boost to the region.

“Quicker journey times, reduced congestion and cleaner air will benefit people locally and unlock growth in the tourism industry,” he said. The planning process for the scheme will begin next, and construction could start in 2021 and take five years.

The culture secretary, Karen Bradley, said: “This investment will help make the visitor experience much more enjoyable. With over 1 million visitors a year, Stonehenge is one of the jewels in the UK’s crown and it is important that we preserve it for generations to come.”

The government said the tunnel would be at least 1.8 miles long and would follow the existing route of the A303 but a further 50 metres from the site than was proposed earlier this year. It said the modification meant the tunnel would avoid important archaeological sites and would not intrude on the view of the setting sun from Stonehenge during the winter solstice.

Building either shorter or longer tunnels has been considered. A shorter one would succeed in making Stonehenge more tranquil but the siting of the portals – the entrances and exits – would damage other important monuments within the world heritage site. A longer one is seen as prohibitively expensive.

A tunnel of 1.8 miles is being presented by the government as a good balance, delivering benefits to the site but still affordable.

One of the main benefits would be that at the eastern end the road dives underground before a feature called the Avenue, believed to be a ceremonial approach to Stonehenge from the river Avon. This would allow visitors and pilgrims, for the first time in decades, to safely follow the route their forebears took.

Organisations including the site’s custodians, English Heritage, and the landowner, the National Trust, were concerned that under plans released earlier this year the western portal was too close to a neolithic and early bronze age feature called the Normanton Down barrow group.

The two bodies, plus Historic England, backed the government’s new tweak. They said: “We believe the amended route can, if designed and located with the utmost care, deliver a lasting legacy for the world heritage site and restore peace and tranquillity to the Stonehenge landscape. This is a once-in-a-generation opportunity to reunite this ancient landscape which is currently severed by a huge volume of road traffic.

“The route announced will ensure the winter solstice alignment will be unspoilt by lights and traffic from the road. We also want to see the globally important archaeology protected, the settings of the ancient burial sites respected and the views between those sites restored. It is now critical to ensure that the benefits of this new route can be realised through careful design and mitigation of archaeological risks, particularly at the western portal of the tunnel and the approach road.”

The AA welcomed the announcement. The motoring organisation’s president, Edmund King, said: “Stonehenge’s ancient circle, created by the can-do attitude of prehistoric builders, has become a monument to modern-age indecision where plans and consultations to ease traffic congestion have gone round and round.

“This improvement is long overdue, not only for the millions of holidaymakers heading west but also for the essential businesses that use this strategic route. Reducing congestion near Stonehenge will greatly benefit the environment and the economy.”

The Stonehenge Alliance expressed anger, especially in light of concern from the heritage body Unesco over the plans for a tunnel. The alliance’s honorary secretary, Kate Fielden, said: “We are shocked at the indifference to Unesco’s advice. The project needs a complete rethink, not a minor tweak which still threatens major harm to this iconic landscape.”

Fielden said the damage to the site would be huge. “There will be deep cuttings, a four-way highway, faster traffic, signage. The potential risk of loss of Stonehenge’s world heritage status casts shame upon our country and those responsible for caring for our heritage.”

The proposals will now be subject to what is likely to be a lengthy planning process.