Ramsey estuary is to be given protective status as an Area of Special Scientific Interest.
The designation has been confirmed by the Department of Environment, Food and Agriculture.
The move aims to protect the most important area of saltmarsh in the Isle of Man, which is a valuable habitat for wildlife.
But Ramsey commissioners are worried it could stifle development in the town.
DEFA will hold a public meeting later this month to explain its reasons for granting the status.
But consultation continues until October 25, so the designation could still be varied, and potentially withdrawn.
The department says a series of reports suggest the intertidal mudflats deserve protection, as they support a diverse range of plants, birds and invertebrates.
However, the commissioners fear it could scupper plans to develop the harbour, which is a key part of the town's long-term regeneration.
Last month commissioners agreed unanimously that protective measures must not be allowed to have any detrimental impact on the town's economy and the livelihoods of local people.
The public meeting will be at Ramsey Grammar School on September 30, at 7.30pm.

Researching your Manx family history can be a very interesting and rewarding hobby. Trace your roots in the Isle of Man with our helpful of guide.