Ten Syrians have been arrested trying to smuggle 20.5 tonnes of cannabis worth an estimated £150million into Europe hidden in a specially converted cargo vessel underneath massive slabs of granite.

The Cook Island-flagged merchant vessel Jupiter was boarded in the Mediterranean Sea by Italian Finance Police who are responsible for border security and taken under guard to the Port of Cagliari on Sardinia.

Europol confirmed that a major joint operation was launched after they received a tip-off that the freighter had drugs hidden beneath the legitimate cargo.

The 821 packages of cannabis, pictured, were hidden inside a cargo vessel registered in the Cook Islands

The drugs were contained in the forward ballast tank of the Jupiter, pictured, after a 15-day search

The drugs were hidden underneath a legitimate cargo of granite, pictured, which filled the vessel's hold

The drugs were found in a specially hollowed-out area in the vessel's forward ballast tank. In total 821 packages containing some 20.5 tonnes of cannabis was recovered.

Italian law enforcement agencies raided the vessel on the evening of September 26 after Spanish Guardia civil aircraft had kept it under surveillance.

It is understood that prior to its arrest, the vessel had visited Vigo in northern Spain, after crossing the Atlantic before heading south into the Mediterranean.

After arriving in Cagliari, a specialist search team boarded the vessel and spent 15 days combing the vessel which is 97 metres long and 16 metres wide.

Italian searchers sought assistance from French experts who played 'a decisive role' in identifying the illegal activity behind the shipment.

According to Europol the drugs were found 'after a long and extremely complex search'.

A spokesman said: 'The enormous illegal load was found skilfully 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.'

Europol received a tip-off that the vessel, pictured, was being used to transport a large amount of illegal drugs