The Government approved a £1.7 billion scheme to overhaul eight miles of the A303 near to the world-famous landmark, including a controversial two-mile tunnel, in July.
Unesco says the World Heritage Site (WHS) could be placed on the danger list if the plans go ahead, but ministers argue the scheme is needed to tackle congestion around the Wiltshire monument.
In a report, Unesco said it has been “clear and consistent that the proposed A303 improvement scheme should not proceed in its current form.”
“The currently proposed western portal and associated dual carriageway within a cutting would have significant and inappropriate adverse impacts on the physical and visual integrity of the property”, it added.
Unesco has asked the Government to provide a “state of conservation” on Stonehenge, and warned the site could be placed on the ‘List of World Heritage in Danger’.
In a letter granting approval to the scheme in July, the Department for Transport said it was satisfied “harm on spatial, visual relations and settings is less than substantial and should be weighed against the public benefits”.
David Bullock, National Highways’ Project Manager for the scheme said: “It is very much a scheme objective to conserve and enhance the World Heritage Site and this is being achieved through close collaborative working with heritage groups, the independent A303 Scientific Committee, and our archaeology contractors, who have an extensive track record of work in connection with the Stonehenge landscape.
“We have taken a lot of care to get to this point, and we will continue to work with the Heritage Monitoring Advisory Group and experts within the Scientific Committee to ensure the scheme is delivered with heritage and the Outstanding Universal Value of the World Heritage Site at the heart of every decision made.
“We remain confident this scheme is the best solution for tackling a long-standing traffic bottleneck, improving journeys, bringing much-needed relief to local communities, boosting the economy in the south-west, while returning the Stonehenge landscape to something like its original setting.”
Campaigners last month presented a petition with 225,000 signatures from 147 countries to Unesco in protest over the plans.
“We urge UNESCO to remain strong in the face of the UK Government’s indifference to this iconic British heritage site,” said the Stonehenge Alliance Campaign.
The Save Stonehenge World Heritage Site (SSWHS) group last month also launched a legal bid to challenge the scheme, saying it was irrational to give “no weight” to the possibility that Stonehenge could be delisted as a World Heritage Site if the scheme went ahead.