Environmental groups warn that proposals to construct 240 turbines in the Bristol Channel threaten hundreds of harbour porpoises.
Conservationists campaigning against the Atlantic Array wind farm have called on ministers to designate the site as a special conservation area, which could prompt planning inspectors to refuse permission.
The Department for Environment, Food and Rural Affairs (Defra) confirmed it is considering new locations to designate as conservation areas.
RWE wants to build the turbines off the North Devon coast eight miles from Lundy Island.
However, the plans have met strong opposition from residents and environmental and heritage groups.
Now it has emerged that RWE estimates in a planning submission that 860 harbour porpoises may be disturbed by noise from pile drivers.
Denise Parker, of the Porthcawl Environment Trust, said: “This is a breeding site and a resting place for the harbour porpoise, so we are very concerned.”
Natural England said the developer had not yet shown that the scheme would not have an “adverse impact” on mammals.
The Planning Inspectorate has also received hundreds of submissions from individuals on the plans.
“The effect on marine life is unknown. I am concerned for the harbour porpoises which are supposed to be protected by EU legislation,” said Rachel Margaret Snaith.
Gareth Thomas, a retired vet living on the Gower coast in South Wales, wrote: “Of the mammals affected, the Harbour Porpoise is the most likely victim, for the local population, centred on Swansea bay, is known to use this area for feeding and recreation.
“It is anticipated that its main threat will be water-borne noise from the operations, particularly in the construction phase, which may last a decade.”
RWE says its environmental assessment concludes that construction noise would have only a “minor adverse” effect.
A Defra spokesman said: “We are working to identify further sites to protect this important species in line with EU requirements.”