The possible seizure of the ship, the Stena Impero, in the Strait of Hormuz, was the latest episode in three months of escalating tensions between Iran and the West. It came a day after the United States said it had downed an Iranian drone in the area, which the Iranians denied.
There were also unconfirmed reports on Friday that a second British-owned tanker may have been seized.
Britain and Iran have been embroiled in a dispute for the past few weeks over Britain’s seizure of an Iranian tanker near Gibraltar. Iran had vowed to retaliate.