The ruling is included in an Advisory Opinion issued by the International Tribunal for the Law of the Sea (ITLOS) on the application of the West African Sub Regional Fisheries Commission (SRFC).
The commission represents fisheries from Cape Verde, The Gambia, Guinea-Bissau, Mauritania, Senegal and Sierra Leone.
The Advisory Opinion stated that countries have a duty of due diligence to ensure that fishing vessels flying their flag do not engage in IUU fishing in the waters of other countries and can be held liable for breach of this duty.
It also holds that the European Union can have the same duty as a state, when it is the party to hold fisheries access agreements with other states.
According to the WWF, IUU fishing in coastal waters costs the world between $10-20 billion annually.
West African waters are believed to have the highest levels of IUU fishing in the world, representing up to 37 per cent of the region’s catch.
WWF International Marine Programme Director John Tanzer said: “This is a very welcome ruling that could be a real game changer.
“No longer will we have to try to combat illegal fishing and the ransacking of coastal fisheries globally on a boat by boat basis.”
The due diligence obligation means that flag States will have to take enforcement actions to ensure their vessels comply with the laws of SRFC member states.
More information on the Advisory Opinion ruling is available here.