Environmentalists from both sides of the border are in Brussels to meet MEPs from the European Parliament's Brexit steering committee.
The group is calling for robust enforcement of laws protecting habitats and species after the UK leaves the EU.
They will argue that different protections in Northern Ireland and the Irish Republic are the "single greatest environmental risk" posed by Brexit.
Both north and south, 650 pieces of EU legislation concern the environment.
Campaigners are concerned that if legislation covering Northern Ireland is watered down, work to conserve species and habitats could be undermined.
"Plant and animal species do not recognise the existence of a border," said Patrick Casement, chairman of Northern Ireland Environment Link, who is part of the delegation.
"Many of these species are currently at risk of extinction on the island of Ireland and any diluting of protection will place them in further danger."
A further concern is that without the oversight of the European Court of Justice there could be a weakening of enforcement of laws on conservation.
Michael Ewing of Environmental Pillar based in the Irish Republic, said the entire focus of the Brexit talks had been on the economy, not the environment.
He said it was vital that the island of Ireland and its waters were treated as a single unit and "mechanisms exist to effectively manage cross border environmental issues post Brexit".