Acting on a tipoff, Royal Gibraltar Police located five inflatable boats loaded with hashish resin in the contested waters around Gibraltar, a tiny British territory on Spain's southern tip.

When police neared one of the boats, its occupants started to throw their cargo overboard in an attempt to lighten their load and pick up speed, Royal Gibraltar Police said in a statement.

The boat left Gibraltar's territorial waters and the chase was then taken up by Spain's Guardia Civil police force but the traffickers managed to escape, it added.

Authorities retraced the route of the chase and picked up 34 bales of hashish resin — Gibraltar police and customs recovered 27 bales and the Guardia Civil another seven.

The drugs weighed around 800 kilos (1,800 pounds) and are estimated to have a street value of £4,050,000 ($4,968,000/€4,762,000).    

“This highlights the close cooperation that exists not just among local law enforcement agencies, but also with the Guardia Civil in combating crime, particularly drug trafficking, across the Straits of Gibraltar,” the statement said.

Madrid ceded Gibraltar in 1713 under the Treaty of Utrecht. The territory remains a source of British-Spanish tensions, with Madrid long claiming it should be returned to Spanish sovereignty.

While Spanish and Gibraltar police cooperate regularly at the grass roots level, there are often flare-ups between the two sides over jurisdiction and sovereignty.

Gibraltar says Spanish vessels regularly stray deep into its territorial waters and Britain's Foreign Office has attacked such incidents as provocative.    

In the most serious recent incident, a British navy patrol boat fired flares to warn off a survey ship operated by Spain's Oceanographic Institute which had entered disputed waters.