Britain said it was urgently seeking further information after reports that a British-flagged tanker, the Stena Impero , had taken a turn into Iranian waters.
"We are urgently seeking further information and assessing the situation following reports of an incident in the Gulf," a spokesman for the Ministry of Defence said on Friday.
A Cobra meeting was taking place between officials from the Foreign Office, Ministry of Defence and other Government departments on Friday night to determine the UK's response.
A Whitehall source said: "We are making contact at increasing levels of seniority with the Iranians to try to establish whether reports coming out of the Middle East are accurate.
"It looks on the face of it as though the Iranian Revolutionary Guard have boarded and taken a UK-flagged ship.
"It appears to be linked to events around the Grace 1 tanker."
More to follow...