Environment Minister Arlene Foster has refused an application for a privately funded visitors' centre at the Giant's Causeway.
Proposals for the Giant's Causeway have caused a political row
Mrs Foster told the NI assembly that she saw some merits in a proposal by developer Seymour Sweeney but had to turn it down on planning grounds.
Last September, the DUP minister said she was "minded" to approve the plans.
But, on Tuesday, she said the proposal would have an "adverse impact on the World Heritage Site".
"I believe it could adversely affect the character of the area," she said.
"There are serious doubts that the proposed development would adequately integrate into the landscape and it would add to the spread of development at this sensitive location."
Following the minister's announcement, a spokesman for Mr Sweeney's company, Seaport NI Ltd, said everyone concerned with the application was "naturally disappointed by the minister's announcement".
"We firmly believe that the scheme proposed, combined with the additional flexibility for infrastructural improvements afforded by our land ownership in the vicinity, addresses all the social, economic and environmental issues presented."
The spokesman added that until company representatives had time to assess the Mrs Foster's written determination, it would be "inappropriate to comment further".
The Causeway attracts nearly 500,000 visitors a year.
The previous visitors' centre on the site burned down in April 2000.
Renowned for its polygonal columns of layered basalt - resulting from a volcanic eruption 60 million years ago - it is the only World Heritage Site in Northern Ireland.