LUXEMBOURG (Reuters) - The European Union’s fisheries deal with Morocco should be declared invalid because it includes Western Sahara, an adviser to the EU’s top court said on Wednesday in the latest legal opinion on trade ties involving the disputed territory.

Western Sahara has been contested since 1975 when Spanish colonial powers left. Morocco claimed the territory as it own and fought the 16-year war with the Polisario Front independence movement which established its self-declared Sahrawi Arab Democratic Republic.

The United Nations says the region has a right to self-determination and campaigners have sought to challenge the EU’s trade deals with Morocco in the courts because they include the desert region.

Wednesday’s opinion by the European Union Court of Justice’s Advocate General Melchior Wathelet came in response to British-based campaigners who said Britain was wrong to uphold the EU-Morocco fisheries deal. Britain asked the ECJ for advice.

“The fisheries exploitation by the EU of the waters adjacent to Western Sahara established and implemented by the contested acts does not respect the right of the people of Western Sahara to self-determination,” Wathelet said.

Wathelet also said his opinion was based on the condition in international law that any wealth gained from exploiting Western Sahara’s rich marine life should benefit its people.

The sparsely populated stretch of desert has offshore fishing as well as phosphate and possibly oil reserves.

The opinion, if followed up by a ruling from the ECJ, is likely reignite a diplomatic dispute between Brussels and Rabat that erupted in 2016 when the lower General Court ruled that the EU trade deals dating from 2000 and 2012 with Morocco were void.

Morocco briefly suspended contact with the EU to lodge a legal appeal.

The EU’s top court later overrode the lower court’s decision, saying the trade accords were valid but could not include Western Sahara, appearing to sidestep the issue.

But the court still said that some EU fisheries in disputed coastal waters would be in violation of the ruling, as the court signaled deals signed with Morocco could not include Western Saharan resources as its inhabitants had not agreed to that.

The court adviser’s latest opinion appeared to support that.

Wathelet said most of the waters fished in the EU-Morocco trade pacts were along Western Sahara’s coast. On that basis Western Sahara should be paid for the fish exported to the EU.

“The fisheries agreement does not contain the legal safeguards necessary for the fisheries exploitation to be for the benefit of the people of Western Sahara,” he said.