The government is being urged to make a quick decision on a £193m scheme to divert roads away from Stonehenge.
English Heritage wants to build a £67m visitor centre
It had been hoped the project, which includes building a tunnel for the A303 near the ancient monument in Wiltshire, would go ahead this year.
But English Heritage, which manages Stonehenge, fears delays in getting a planning report to the government means work may not now start until 2006.
An English Heritage spokeswoman said: "We are losing momentum."
The organisation has ambitious plans to build a £67m visitor centre, just outside the 26 sq km Stonehenge site.
Its proposals are being scrutinised by Salisbury District Council planners and a decision could be made by the end of the summer.
But even if the plans get the green light, work cannot start until the government agrees to the separate road scheme.
The current road system would not be able to cope with the traffic using the new visitor centre.
"We have put an enormous amount of work into the planning application for the visitor's centre," the English Heritage spokeswoman said.
"It's crucial we have a decision on the roads as soon as possible,"
English Heritage had been expecting a decision on the Highways Agency's road plan early this year.
When a three-month public inquiry into the scheme finished in May 2004, the planning inspector said he expected to have his report on Secretary of State for Transport Alistair Darling's desk by September.
But the report did not reach Whitehall until the end of January.
It is feared this delay, coupled with the possible distraction of a General Election, could mean the government may not make a decision until late summer or early autumn.
The English Heritage spokeswoman said that even if both projects got the go-ahead this year, work would probably not start on either until the spring of 2006.
The group's concerns follow an open letter to Mr Darling from Fiona Reynolds, director general of the National Trust, in which she urged the government not to let the road project become obscured in "bureaucratic long grass".
The National Trust owns much of the land within the Stonehenge site.
A Department for Transport spokesman told the BBC that no date had been set for the government's recommendations on the road scheme.