Stormont has failed to protect Strangford Lough properly for two decades, campaigners have said.

Horse mussel reefs have been destroyed and habitat for sealife ruined, Ulster Wildlife Trust added. The trust has raised the damage with the European Commission.

Heather Thompson, the trust's chief executive, said: "Despite promises to Europe in 2005 that NI would finally take serious action to restore the reefs to their former glory, the Department of Agriculture and the Department of Environment have failed to deliver, resulting in continued breach of the EC Habitats Directive, and continued decline of this vital natural resource."

The Assembly is due to debate the situation at the lough.

Ms Thompson added: "Whilst we were encouraged today to hear that the Assembly recognises the need to secure a sustainable future for Strangford Lough, unfortunately marine life continues to suffer as a result of mismanagement.

"We are urging the Executive to take prompt and decisive action, and to send a clear message to Europe that we take seriously our most basic of responsibilities to look after the environment, before the taxpayer is left with a bill of millions of euros."

The trust's main concern is the destruction and deterioration of horse mussel reefs in the lough, which provide vital homes, spawning grounds and nurseries for hundreds of creatures, including commercially fished species.

The organisation believes options for restoring this underwater habitat are limited and that the Stormont Executive needs to bring in an extensive non-disturbance zone, limiting the area available for diving, fishing and anchoring to give the reefs a chance to recover.

Representatives from the European Commission are to meet the Department of Environment and the Department of Agriculture and Rural Development on Tuesday to discuss the trust's complaint.

Ms Thompson said: "We have not taken the decision to seek help from Europe lightly. Despite years of campaigning on this issue, unfortunately we have been left with no other option than to ask Europe to ensure that our government departments get their act together."