Stonehenge tunnel benefits 'uncertain' says Audit Office
There are "risks and uncertainty" over a road tunnel near Stonehenge and its benefits are "inherently uncertain", a scrutiny body said.
The government wants to build a tunnel past the monument as part of a £1.6bn plan to upgrade the A303.
However, the National Audit Office (NAO) estimates the likely cost as £1.9bn, and says it "must deliver value for taxpayers".
Highways England says the route will cut congestion and boost the economy.
Work is due to begin in 2021 with an expected opening date of 2026.
The NAO estimates the project will only deliver £1.15 in benefits for every £1 spent.
Auditor General Amyas Morse 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."
The NAO also warned the project poses "geological and archaeological risks", and said Highways England must ensure it can "support the project throughout its life".
- Q&A: Stonehenge tunnel row
- Hole dug at Stonehenge site a 'travesty'
- Road tunnel will 'protect' Stonehenge
The government wants to build the tunnel to hide the busy A303, but opponents claim it could destroy archaeological treasures and scar the landscape.
The International Council on Monuments and Sites (Icomos) described the plans as "severely flawed" while The Stonehenge Alliance - a campaign group which includes archaeologists and environmental campaigners - said the work threatened the area's "fragile archaeology".
Highways England said the "vital route" would cut congestion and boost the economy, and would "restore the tranquil environment and setting of the monument".
The Department for Transport said the road upgrade would "improve 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.
"Across the South West, we are investing £2 billion to improve roads, on top of £133 million for Bristol, North Somerset and Gloucester to introduce the MetroBus rapid public transport."
A public consultation into the scheme ended in April 2018.
Highways England submitted a development consent order to the Planning Inspectorate in November.
The agency has set a six-month timetable in order to examine the proposals.