Spanish police operating in international waters near the Canary Islands have impounded what they say is the country's largest-ever haul of cocaine.

The union representing members of the Civil Guard, a division of the police that deals with issues such as border security, told Agence-France Presse that 23 people — mostly Filipino, Angolan, and Dutch nationals — had been arrested after the interception of a ship sailing from Sierra Leone's capital Freetown to the Libyan port of Benghazi.

The vessel was described as being "completely stuffed" with bales of cocaine, thought to weigh up to 40 tons.

The union called the bust a "historic blow to drug trafficking".

Sky News reported that the 90-meter, Comoros Islands-registered vessel was intercepted off the coast of southern Morocco's Western Sahara region, close to the city of Dakhla, and escorted to the Canary Islands port of Las Palmas on Sunday night.

Although Libya was the ship's final destination, it is believed the cargo was intended to be split up into smaller portions and dispersed at sea, because, based on similar previous operations, such a large amount of drugs going into a port would not make sense.

"Many vessels would be needed, different ports, because such an unloading in a single port would arouse a lot of suspicion," said a representative of the AUGC union. "So, the operation is probably coordinated through international networks."

Because of its historic and cultural links to Latin America, and its geographical proximity to Morocco, which is a leading producer of cannabis, Spain has long been one of the main entry points for illegal drugs coming into Europe.

A court order means there are limits on what specific details of the case can be made public, but, if confirmed, the haul would dwarf what had previously been regarded as major Spanish police successes in the fight against drug smuggling.

In October, police made nine arrests and seized 6.5 tons of cocaine in another bust off the Canary Islands. And in January, what was the previous largest seizure at sea was made, when nearly 10 tons of cocaine was intercepted.

While declining to go into detail, Spain's Interior Minister Fernando Grande-Marlaska said the police success "was one of the biggest, not only nationally but internationally".