Iran warned Britain that it "should be scared" that a British ship will be seized in the Gulf in a tit-for-tat retaliation.

Tensions continued to rise as a senior cleric renewed the threat of a revenge attack after the Royal Marines boarded a ship taking Iranian oil to Syria in breach of EU sanctions.

A general in Iran's Revolutionary Guard had already threatened to seize a British ship on Friday after marines took control of Grace 1, a tanker owned by a Dubai-based company with ties to Tehran, was believed to be carrying two million barrels of crude oil to Syria Bashar Assad’s regime.

Fears of a reprisal intensified on Saturday when Mohammad Ali Mousavi Jazayeri, a member of the powerful Assembly of Experts clerical body said: “I am openly saying that Britain should be scared of Iran’s retaliatory measures over the illegal seizure of the Iranian oil tanker.”

An editorial in the Kayhan Daily, an Iranian newspaper, said the “UK must be made to regret its seizure of our oil tanker. Those in our government who call for restraint are at best stupid and at worst the fifth column of Britain.”

For much of Saturday morning all eyes were on a British supertanker stalled for several hours in the Gulf, with many observers fearing it had fallen victim to an attack by the Iranian Revolutionary Guard.