Britain said it was urgently seeking information about the Stena Impero after the tanker, which had been heading to a port in Saudi Arabia, suddenly changed course after passing through the Strait of Hormuz at the mouth of the Gulf.
A second tanker, the British-operated Mesdar, also made a sharp change of direction in the Gulf around the same time. But the Guards made no mention of a second vessel.
Relations between Iran and the West have become increasingly tense since British naval forces seized an Iranian tanker in Gibraltar on July 4 on suspicion of smuggling oil to Syria in breach of European Union sanctions.
(Production: Edward Baran)