World Heritage status could be withdrawn from the Jurassic Coast if a proposed wind farm goes ahead.
Unesco has written to the government about the Navitus Bay project, which is planned off the coast of Dorset, Hampshire and Isle of Wight.
It says the turbines would have an adverse impact on important views.
Navitus Bay said it was aware of the letter but added the comments contradicted the independent impact assessment already carried out.
The Jurassic Coast, which covers 96 miles (155km) of coast in Dorset and East Devon, is the only natural heritage site in England.
Unesco commissioned its own impact study and is concerned the 194 proposed turbines will dominate the view from Dorset across to the Isle of Wight.
The letter says: “Any potential impacts on this natural property (the Jurassic Coast) are in contradiction to the overarching principle of the World Heritage Convention.
“The property will change from being located in a natural setting largely free from human-made structures to one dominated by human-made structures.”
The government wrote to Unesco in February saying the impact was minimal. The final decision will be made by the government’s planning inspectorate.
Mike Unsworth, project director at Navitus Bay, said: “We will be seeking further clarification as the interim comments [in the letter] are not aligned with the independent impact assessment or the conclusion of DCMS (Department for Culture Media and Sport).”
He said that the DCMS stated: “‘Our overall conclusion, on the basis of the evidence presented so far, is that while the proposed wind farm will have some impact on the World Heritage property, there should be no significant impact on outstanding universal value’.”
The plans were officially submitted in April amid claims the wind farm would contribute £1.6bn to the economy over 25 years and would generate power for up to 700,000 homes.
Unesco, a branch of the United Nations, awards world heritage status to cultural and natural sites.