British Royal Marines, along with Gibraltar port and law enforcement agencies, seized the oil tanker Grace 1 which was carrying crude oil to the Baniyas Refinery in Syria, the BBC reported.
The refinery is subject to European Union sanctions against Syria, Picardo said, adding that there was a reason to believe that the ship was carrying crude oil to the Syrian refinery.
The Baniyas refinery in the Syrian Mediterranean port town of Tartous is a subsidiary of the General Corporation for Refining and Distribution of Petroleum Products, a section of the Syrian Ministry of petroleum.
"That refinery is the property of an entity that is subject to EU sanctions against Syria," Picardo said.
"Be assured that Gibraltar remains safe, secure and committed to the international, rules-based, legal order," he said, thanking the police, customs and port authorities for their involvement in detaining the ship.
According to the BBC, a team of about 20 marines from 42 Commando were flown from the UK to Gibraltar to help seize the tanker, at the request of the Gibraltar government.
A defence source cited by the broadcaster described it as a "relatively benign operation" without major incident.
The EU says the refinery provides financial support the Syrian government, which is subject to sanctions because of its repression of civil since the start of the uprising against President Bashar al-Assad in 2011.
The refinery has been subject to EU sanctions since 2014.
( With inputs from IANS )