British Royal Marines boarded the ship, Grace 1, off the coast of Gibraltar on Thursday and seized it over accusations it was breaking sanctions by taking oil to Syria.
"Her Majesty's Government of Gibraltar can now confirm, after having received the results of comprehensive laboratory testing, that ... the Grace 1, which was detained in the early hours of Thursday morning, is loaded to capacity with crude oil," the government said in a statement.
"The results of these tests... contradict the statements of some commentators from outside the jurisdiction who had speculated that the cargo on the vessel was not crude."