The Manx Wildlife Trust (MWT) has formally opposed plans to redevelop Ramsey Bay.
The organisation said its decision comes after extensive research and discussion with developers.
Ramsey Marina Limited (RML) wants to develop a 400-berth dock, a five-star hotel and a yacht club near the town's harbour.
The company said the £100 million scheme could provide hundreds of new jobs and create a thriving marine ecosystem.
It would see the creation of 200 town houses and apartments, commercial units, bars and restaurants.
But the Manx Wildlife Trust said the development would cause ecological damage and "undermine legally-protected wildlife habitats".
A spokesman added that the proposals would conflict with the Isle of Man's UNESCO Biosphere status.
You can read MWT's full statement here and find out more about the development here.
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