The economic case for building a road tunnel near Stonehenge relies on "uncertain" benefits, the public spending watchdog has warned.
A report by the National Audit Office (NAO) raised doubts over the estimated £1.15 benefit for every £1 spent.
The Department for Transport's (DfT) controversial proposed upgrade of the A303 - which includes a two-mile long tunnel beneath the Stonehenge World Heritage Site - is forecast to cost between £1.5 billion and £2.4 billion, the NAO said - potentially putting the project at risk.
The road, which currently passes the stones, often suffers from severe congestion as it is a popular route for motorists travelling between London and the South West.
But the high cost of building a tunnel compared with widening the road means 73% of the financial benefits are heritage-related. These were calculated by asking people what they would be willing to pay to remove the road from the site.
These benefits are "inherently uncertain" because they are based on a hypothetical situation and are difficult to measure, the watchdog warned.
NAO head Sir 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 project, to be carried out by the Government-owned company Highways England, is due to be completed by December 2026.
The NAO noted that uncertainty over how the scheme will be funded has led to delays.
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It was due to be publicly financed until the Treasury instructed the DfT to use private funds in October 2016, which pushed back the start of construction by 21 months to December 2021.
In October last year, Chancellor Philip Hammond cancelled future public-private partnerships, including the Stonehenge project.
The NAO stated that the Government says it is committed to the A303 scheme but it is unclear how it will be funded.
A DfT spokeswoman 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."
Archaeologists responsible for many of the recent discoveries in the Stonehenge landscape have hit out at the road tunnel plan, claiming it would destroy important archaeological remains.