Spanish police said Wednesday they had seized more than 30 tonnes of hashish stashed onboard four sailing boats in its biggest-ever bust of such drugs at sea. 

The four boats were intercepted in the Atlantic in the framework of a four-day operation which ended on Monday with the arrest of nine people of Bulgarian and Russian origin, a police statement said. 

Two of them, both "luxury vessels", were stopped off the coast of Fuerteventura in the Canary Islands, while a third was seized near the Strait of Gibraltar and the fourth off the coast of Mauritania, south of Morocco. 

Sailing under British, German and Dutch flags, the vessels belonged to what police said was "the biggest criminal organisation dedicated to trafficking hashish by sea".

Since the beginning of the year, Spanish police have seized nearly 35 tonnes of hashish belonging to this organisation, which is run by Bulgarian nationals, along with six of their sailing boats, valued at some 5.0 million euros. 

Spanish security forces staged the operation after coordination with police in Britain, the Netherlands, Greece and Italy, and the help of Europol, the European policing agency. 

Spain is one of the main gateways for drugs coming into Europe and criminal gangs from eastern Europe are well established within the country. 

Earlier this month, Spanish police seized a Croatia-flagged sailing boat off the Canary Islands carrying nearly a tonne of cocaine destined for eastern Europe with a street value of more than 40 million euros, prompting officials in Spain and Croatia to speak of a "new African route" for drugs.