Environment Minister Arlene Foster has refused developer Seymour Sweeney planning permission for his proposed Giant's Causeway visitor centre, she announced today.
The Minister revealed her decision to the Assembly, reversing her earlier position of being "minded" to approve the property tycoon's blueprint.
The verdict followed a series of separate discussions between senior planning service officials and Mr Sweeney, the National Trust and Moyle Council.
These talks were aimed at addressing concerns on key issues, including the potential impact of the developer's scheme on the Causeway's world heritage site status.
Mrs Foster today said: " My officials' discussions with the key stakeholders have now been completed and I have received their report on those, as well as their reassessment of the application in light of the outcome.
"I am convinced that the proposal as it stands would have an adverse impact on the world heritage site as I believe that it could adversely affect the character of the area.
"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.
"Although I still see merit in the proposal, I consider that this does not outweigh the planning concerns that I continue to have about it and which clearly are not capable of being addressed to my satisfaction.
"I have therefore concluded on balance and on further reflection that the application should be refused."
Mrs Foster came under fire last September for announcing that she was "of a mind" to grant Mr Sweeney planning permission.
Her DUP Ministerial colleague Nigel Dodds immediately shelved a Government visitor centre scheme, involving the National Trust and Moyle Council.
Political opponents were today hailing a "U-turn" by Mrs Foster. But her party was poised to stoutly defend her position at the Assembly.
DUP sources are arguing that the Minister has galvanised debate on the fact that a permanent replacement has still to be provided for Causeway visitor facilities damaged by fire in 2000.
The controversy since last September has been intensified by the fact that Mr Sweeney is a DUP member and has enjoyed sustained lobbying support from DUP MLA Ian Paisley Jnr.
It was revealed in recent weeks that Mr Paisley Jnr had even raised the Causeway issue at the 2006 St Andrews talks - on a constituency " shopping list" considered by Prime Minister Tony Blair.
Mrs Foster has stressed that she was unaware of Mr Sweeney's party affiliation when she made her "minded" announcement and had not been lobbied on the application by anyone.
She spoke positively last September of the possibility of the developer's new centre replacing the existing facilities run by the National Trust and Council.
Her original " minded" stance apparently stemmed from a hope that a three-way agreement could be sealed, involving the Trust and Moyle Council coming on board Mr Sweeney's development scheme.
But it soon became clear in the talks involving planning officials that there was no prospect of such a deal.
The Trust has stated that the scale and location of Mr Sweeney's proposed centre would breach guidelines drawn up by Unesco, the UN body responsible for the Causeway's world heritage status.
Moyle Council and the Trust have moved in recent months to take charge of the alternative visitor centre scheme shelved by Mr Dodds' department.
A planning application is expected to be submitted on that project later this year.