An appeal against a £160m upgrade of the A6 road in Co Antrim will not be heard by the Supreme Court, it has been announced.
A section of the route at Toomebridge skirts Lough Beg, an internationally recognised bird sanctuary with EU protection.
Environmentalist Chris Murphy brought the application under the EU Habitats Directive legislation, which requires the Government to give appropriate protection to specially designated sites.
He claimed a proper impact assessment was not carried out.
The Department for Infrastructure said the required checks had been done.
Mr Murphy had previously lost cases in the High Court and Court of Appeal in Northern Ireland.
The decision of the Supreme Court not to hear the appeal effectively ends any domestic legal challenge to the scheme.
However, Mr Murphy vowed to take his case to the European Court of Justice and requested that the authorities continue to suspend all work on the contested 2.5 mile section of road.
"My application was refused today but I remain 100% convinced that the law is firmly on the side of these incredible wetlands, which are an irreplaceable habitat whose value to man and nature has not been given proper consideration," he said.
"It is simply perverse to allow this construction to go on and cause irreversible harm to a Special Protection Area when many alternative routes exist.
"The government is required in law to give these areas the strictest protection."
Sinn Fein MLA Philip McGuigan welcomed the court's decision, calling the A6 upgrade a "key infrastructural project to help promote regional balance".
The Department for Infrastructure said it would "proceed with the whole scheme, taking into account the constraints identified in the Environmental Statement".