The illegal narcotics were found hidden in the hull of the 97 metres long and 16 metres wide Jupiter cargo ship.
A Europol spokesman said Italian police confiscated 821 packages of cannabis weighing a total of 20.5 tonnes in a ballast tank in the bow of the ship.
The ship’s captain and nine crew members, all Syrian, were arrested, the spokesman said.
The Cook Islands Ministry of Foreign Affairs and Immigration confirmed to CI News that the ship was registered under the Cook Islands flag.
The ministry also confirmed the Jupiter had been detained by Italian authorities after a large quantity of illegal narcotics had been discovered on board.
A ministry spokesman said a number of crew had been arrested, including the captain of the vessel and the ministry was now considering removing the vessel from the ships’ register.
“Boarding of the vessel took place in compliance with Article 17 of the United Nations Convention against Illicit Traffic in Narcotic Drugs of 1988 and with the full knowledge and authorisation of the government of the Cook Islands, whose cooperation was acknowledged and appreciated,” the ministry said in a statement.
“The cooperation of the local agencies with their Italian counterparts was important in ensuring a successful outcome to the operation.
“The government of the Cook Islands is continuing to cooperate with the Italian authorities, particularly in respect of the remaining crew onboard. It is considering its options further with regard to the vessel, including its possible deregistration from the ships’ register.”
Italian finance police, who help oversee border security, acted on a tip and boarded the ship in the Mediterranean Sea, diverting it to the port of Cagliari in Sardinia, Italy.
Police then spent more than 18 days searching before finally finding the drugs.
During the search, police used cranes to move enormous blocks of granite carried by the Jupiter cargo ship.
A Europol spokesman said: “The enormous illegal load was found skillfully stashed in a large false bottom under the hold of the vessel with its only access hidden by hundreds of tonnes of granite being used as legal cargo.”’
According to the website vesselfinder.com, the ship’s last port of call was Vigo in Spain.