An oil embargo against Iran has been agreed as part of EU sanctions over its nuclear programme.
Foreign Secretary William Hague called the measure part of "an unprecedented set of sanctions."
"I think this shows the resolve of the European Union on this issue," he said
Diplomats say the measures, adopted by the EU's 27 foreign ministers, include an immediate embargo on new contracts for crude oil and petroleum products while existing ones are allowed to run until July.
Iran says its nuclear programme is exclusively for peaceful purposes. But many international officials fear the country is trying to develop nuclear weapons.