The letter, submitted to the Examining Authority, is backed by Kent Wildlife Trust, CPRE Kent, the Suffolk Preservation Society, British Divers Marine Life Rescue and several community groups across Suffolk and Kent.
Conservative MP Sir Roger Gale has also lent his support.
Sea Link proposes a high-voltage connection between Suffolk and Kent to strengthen the UK’s electricity grid.
But campaigners argue the Development Consent Order (DCO) is “riddled with errors and omissions”, including missing ecological surveys, contradictory documents and a failure to meet statutory consultation duties.
Cables are planned to make landfall at two nationally and internationally protected sites: the RSPB’s North Warren Nature Reserve in Suffolk and Kent Wildlife Trust’s Sandwich and Pegwell Bay Nature Reserve.
Both areas are home to saltmarsh, wetlands and key habitats for protected species.
The plans also include new converter stations at Saxmundham and Minster Marshes – standing up to 28 metres high.
The coalition says trenchless drilling poses “major risks” to these delicate habitats, with no credible safeguards for preventing drilling fluid leakage or other impacts.
Despite repeated requests during the pre-application phase, National Grid submitted the DCO without essential baseline data or mitigation plans.
Emma Waller of Kent Wildlife Trust said: “We fully support the clean energy transition. But renewable energy cannot come at the cost of our natural environment. This DCO is not fit for Examination. Communities and wildlife deserve better.”
More than 500 documents have been submitted with the application, many containing factual errors and inconsistencies.
Campaigners say the volume and quality of material unfairly shifts the burden onto under-resourced local authorities and charities to check the work.
They are calling for the Sea Link application to be withdrawn and only resubmitted once it is legally compliant, environmentally sound and procedurally robust.
Anything less, they say, threatens the integrity of the planning process and the nature it claims to protect.
