Spending watchdog the National Audit Office says there are “risks and uncertainty” around the project being delivered on time and achieving the outcomes that Government hopes it will bring.

The project is forecast to cost between £1.5Bn and £2.4Bn, but is only estimated to deliver £1.15 in benefit for every £1 spent.

NAO head Amyas Morse said: “The tunnel at Stonehenge is currently only just value for money.

“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.”

The 3.3km Stonehenge tunnel is part of a planned improvement to the A303 in Wiltshire between Amesbury and Berwick Down and is set to open in December 2026. This project is one of eight schemes to upgrade the A303 and A358 corridor to the South West of England by the end of the next decade.

There have been delays with the Amesbury to Berwick Down scheme due to decisions over how it will be funded. It was initially to be funded publically, but HM Treasury instructed the Department for Transport in 2016 to use private finance. This, according to the NAO, delayed the start of construction by 21 months.

Last October the Chancellor cancelled future private finance deals. As of February 2019, Highways England is said to have spent £53M on the project and HM Treasury has granted a further £21.5M for pre-construction works.

The NAO says that it is not clear how the project will be funded and that there are risks that Highways England and the Department will need to manage “to ensure the project has a realistic chance of delivering value to taxpayers”.

The planned opening date of December 2026 will mean a very tight construction timetable, it adds, and there are geological and archaeological risks that need to be managed.

The NAO also points out that previous attempts to construct a tunnel have been cancelled due to escalating costs and disagreements between stakeholders.

While Highways England has managed to gain agreement in principle from key stakeholders such as the National Trust and Historic England, other bodies, including the UNESCO World Heritage Committee, have raised concerns, the audit office also said.

(Photograph: Department for Transport)