The government is to re-examine plans for a road scheme aimed at diverting traffic away from Stonehenge after the cost of the project doubled.
The tunnel plan had been identified as the best option for the site
The scheme, which includes building a tunnel for the A303 near the ancient Wiltshire site, was estimated to cost £183m when it was announced in 2002.
But now the government says the project will cost around £470m
A detailed review of the tunnel plan, as well as other road proposals for the site, will now be carried out.
The tunnel scheme had been identified in a government study as the best option to divert traffic away from Stonehenge.
Alternatives included re-routing the A303 away from the historic stones.
A three-month public inquiry finished in May 2004 and the planning inspector's report was submitted to the government in January.
But the Department for Transport (DfT) said on Wednesday that a detailed survey of the tunnel site had revealed two factors which would make the scheme more complicated - and expensive.
The first was the presence of large quantities of soft chalk, and the second a high water table.
"The increase in scheme costs represents a significant change to the basis on which the government originally decided to progress this scheme," roads minister Stephen Ladyman said.
"Our recognition of the importance of Stonehenge as a World Heritage site remains unchanged, but given the scale of the cost increase, we have to re-examine whether the scheme still represents value for money."
Any delays would have an impact on plans for a new vistor centre
A DfT spokesman said the problems would not have surfaced in the original government study.
He added there was a possibility a further public inquiry could be called if the government opted for another scheme.
It had been hoped work would start by early 2006, but the DfT spokesman could give no indication about how long the review would take.
The government will also consider the implications of delaying the project while other A303 improvements are carried out.
Any delays in approving the road plan will have an impact on English Heritage's proposal for a £67m visitor centre at Stonehenge.
Those plans were backed by members of Salisbury District Council's northern area committee on Tuesday and will now go forward to a decisive planning meeting on 26 July.