Iranian cleric Mohammad Ali Mousavi Jazayeri warned Britain on Saturday about Tehran's retaliation for the capture of an Iranian supertanker by Royal Marines in Gibraltar. "I am openly saying that Britain should be scared of Iran's retaliatory measures over the illegal seizure of the Iranian oil tanker," said the cleric, who is a member of the powerful clerical body, the Assembly of Experts. British Royal Marines seized the supertanker Grace 1 on Thursday for trying to take oil to Syria in violation of EU sanctions, a dramatic step that drew Tehran's fury and could escalate its confrontation with the West. It comes as British flagged supertanker Pacfic Voyager is passing through the Persian Gulf this weekend.  Tehran yesterday threatened to seize British shipping in the Gulf in a direct response to the dramatic swoop by the marines with helicopters and fast boats in the early hours of Thursday.    Yesterday, Moshem Rezaei said Tehran had a "duty" to respond to the seizure of the Grace 1 over suspicions it was carrying oil to Bashar Assad's Syrian regime in breach of EU sanctions. A detachment of Royal Marines from 42 Commando boarded the vessel on Thursday in a joint operation with the Royal Gibraltar Police. Writing on Twitter, Mr Rezaei, who led the Guard during the 1980s "tanker war" in the Gulf, said: "If England does not release the Iranian oil tanker, the duty... (of Iran) is to respond and seize one English oil tanker." The warning came after Britain's ambassador in Tehran, Rob Macaire, was summoned to the Iranian Foreign Ministry to explain the Government's actions.    In a statement, the Gibraltar government said it had taken the decision to intercept the vessel and was not acting at the behest of any other country. It follows a claim by the Spanish authorities that the seizure of the tanker was made at the request of the United States which had been tracking its movements. "There has been no political request at any time from any government that the Gibraltar government should act or not act, on one basis or another," the Gibraltar government said in a statement. "The decisions of Her Majesty's Government of Gibraltar were taken totally independently, based on breaches of existing law and not at all based on extraneous political considerations."    A spokesman said members of the crew were continuing to be questioned as inquiries continued aboard the ship. The 28 crew members - mainly Indian, Pakistani and Ukrainian nationals - were being interviewed as witnesses and were not being questioned under criminal procedures the spokesman added. The incident has occurred at a time of heightened tensions between the US and its allies and Iran over the unravelling nuclear deal between Tehran and leading international powers, including the UK. Last month President Donald Trump said he had made a last-minute decision to call off air strikes in retaliation for the shooting down by Iran of an unmanned US drone over the Gulf.    Earlier thi