IRANIAN leaders threatened to seize a British oil tanker today unless an Iranian ship held by Royal Marines near Gibraltar is released.

Supertanker the Grace 1 was seized yesterday after it was accused of shipping oil from Iran to Syria in breach of EU sanctions.

Mohsen Rezaei, adviser to Iranian Supreme Leader Ayatollah Ali Khamanei, warned that Tehran would respond to bullies “without hesitation” after the British ambassador had been summoned to answer complaints that this country is engaging in a “form of piracy.”

"If Britain does not release the Iranian oil tanker, it is the authorities’ duty to seize a British oil tanker,” Mr Rezaei said.

Navid Shomali, international secretary of Iran’s communist Tudeh Party, warned that the situation was becoming increasingly volatile and called on the EU and international community to comply with their obligations under the nuclear deal with Tehran.

“Any unwise action or moves from either side could precipitate a chain reaction leading to a war of devastating proportions,” he said.

Mr Shomali called for signatories to the Joint Comprehensive Plan of Action, as the Iran nuclear deal is formally known, “to firmly apply the brakes in this course to all-out conflict.”