Risks and uncertainty surround the government’s plan to build a tunnel beneath the Stonehenge world heritage site, the National Audit Office has concluded.

A report from the NAO casts serious doubt on the whether the £2bn project will represent value for money and whether the work could really be completed by the proposed opening date of December 2026.

It also says Highways England must make sure it can support the project after it is built, flagging up that the costs of operating, maintaining and renewing it are likely to be more than £500m over 60 years.

The NAO report comes at a sensitive time for the plan, coinciding with the start of what are bound to be heated public hearings arranged as part of a six-month examination by a panel of five inspectors.

Many people, from environmentalists and archaeologists to residents and druids, have claimed that the tunnel and other construction work will cause damage to the ancient site.

Plans for a tunnel have been mooted for decades. The Department for Transport believes its construction will improve the experience of visiting Stonehenge, which is currently blighted by the sight and sound of cars and lorries passing close by on the A303, and also reduce congestion and support economic growth.

According to the NAO report, the Amesbury and Berwick Down project, which includes the tunnel, is forecast to cost between £1.5bn and £2.4bn, with a likely cost of around £1.9bn.

The NAO, the independent parliamentary body responsible for auditing government departments, says the project is predicted to deliver £1.15 in benefits for every £1 spent, in part due to the high cost of building a tunnel.

The report highlights concerns over how this figure was arrived at. It says that under the standard method for appraising transport projects, the project would deliver 31p for every £1 spent.

Facebook Twitter Pinterest An image of the planned tunnel. Photograph: Highways England

It says the £1.15 figure was reached because Highways England included a monetary value for “cultural heritage” in the costings. Highways England worked out this value by asking the public how much it would pay to have the road removed from the site.

The report says: “While Highways England used approved methodologies to do this, calculating benefits in this way is inherently uncertain.” It also says the £1.15 figure could slip. “Given our experience of cost increases on projects of this kind, this ratio could move to an even lower or negative value,” the NAO says.

The tunnel project is one of eight schemes to improve a busy route between the south-east and south-west of England. The aim is to upgrade the whole of the A303, which passes close to the stone circle, and the A358 further west.

Highways England believes that if all eight projects are completed, its return on investment on the Amesbury to Berwick Down project will be maximised. But the report argues that not all eight may go ahead.

The report says: “On its own, the Amesbury to Berwick Down project delivers some localised transport and economic benefits, such as reduced congestion in the local area. It will also improve the setting of the Stonehenge monument. However, the project can only create a high-quality route to the south west and unlock the full growth potential in the region in combination with the seven other projects.”

The Amesbury to Berwick Down plans have been delayed because of decisions about funding. The NAO says: “The government says it remains committed to the project, but it is not clear how the project will be funded.”

Amyas Morse, the head of the NAO, said: “The tunnel at Stonehenge is currently only just value for money by the department’s own business case. Based on experience, project costs tend to grow rather than fall, at least in the early years. It will take a very special effort by the department to protect public value up to completion.”

A DfT spokesperson said: “Investing in the south-west is an absolute priority for this government. Upgrading the A303 at Stonehenge will boost the region’s economy and improve its connections with the rest of the country.

“Stonehenge is a site of significant historical value – we have worked closely with heritage groups, including English Heritage and Historic England, to ensure it is protected both during the upgrade of the A303 and in the long-term.”