Britain said on Thursday Iran must abide by assurances it had given that the Grace 1 tanker would not travel to Syria, and said it would not allow Iran or anyone to bypass European Union sanctions.
"We note the Government of Gibraltar has received assurances from Iran that the Grace 1 will not proceed to Syria. Iran must abide by the assurances they have provided," a British foreign office spokesman said in a statement.
"We will not stand by and allow Iran or anyone to bypass vital EU sanctions on a regime that has deployed chemical weapons against its own people.
"There is no comparison or linkage between Iran's unacceptable and illegal seizure of, and attacks on, commercial shipping vessels in the Strait of Hormuz and the enforcement of EU Syria sanctions by the Government of Gibraltar."