Spain and the UK are caught up in a new row over the waters near Gibraltar, a disputed British overseas territory located at the southern tip of the Iberian peninsula.

An Iranian oil tanker on its way to Syria was stopped on Thursday by Gibraltarian authorities with help from British forces in waters that Spain considers to be its own.

We are analyzing the circumstances and seeing how they affect our sovereignty

Acting Foreign Minister Josep Borrell

The move prevented the supertanker Grace 1 from delivering Iranian crude oil to Syria in breach of the European Union’s sanctions on the regime of Bashar Al-Assad.

Gibraltarian authorities issued a release detailing the operation, which was apparently triggered by intelligence information that the US relayed to Britain, implying that the tanker was in British territorial waters.

Madrid will formally complain to London over what it considers to be an incursion into Spanish waters, said a source at the Foreign Ministry. Spain does not recognize any Gibraltarian territorial waters, based on the 1713 Treaty of Utrecht by which Spain ceded the territory during the War of Spanish Succession.

“We are analyzing the circumstances and seeing how they affect our sovereignty,” said Josep Borrell, the acting Spanish foreign minister and the EU’s official nominee to be the next High Representative for Foreign Affairs and Security Policy.

Statement on the detention of the Super Tanker "Grace 1". pic.twitter.com/bzD9WMR6nH — Fabian Picardo (@FabianPicardo) July 4, 2019

According to the official version of events, London alerted Madrid about the presence of the supertanker and warned that “there was going to be an intervention by British forces to detain it in the port of Gibraltar.”

But the seizure did not take place in the port, which is part of Gibraltar’s territory, but further out in waters that Spain considers its own. In spite of it, Madrid made no attempt at halting the boarding operation.

“Spain did not want to interfere because this was about upholding EU sanctions,” said a ministry source. A Civil Guard patrol boat was sent out to monitor the operation.

British diplomatic sources told EL PAÍS that they are convinced Madrid and London are working towards the same goal of ensuring that EU sanctions against Syria are respected.

“It was done in observance of international law, and we have no doubt whatsoever that the government of Spain also supports the sanctions regime, even though we admit that both governments still have a pending dispute over the territory of Gibraltar.”

While Gibraltarian authorities did not mention a US intelligence tip-off, Spain believes this is what triggered the operation. Acting minister Borrell said that Washington had alerted London about the supertanker’s presence in European waters, rather than informing Spain. The foreign ministry did not say whether it will also complain to Washington over the matter.

The waters around Gibraltar have for years been the subject of intermittent disputes involving fishing vessels and patrol boats.