Britain said Iran's seizure of a British-flagged vessel and a Liberian-flagged vessel in the Strait of Hormuz was unacceptable and called for freedom of navigation in the Gulf.
"I'm extremely concerned by the seizure of two naval vessels by Iranian authorities in the Strait of Hormuz," Foreign Secretary Jeremy Hunt said.
"I will shortly attend a COBR (national security) meeting to review what we know and what we can do to swiftly secure the release of the two vessels - a British-flagged vessel and a Liberian-flagged vessel," he said.
"These seizures are unacceptable. It is essential that freedom of navigation is maintained and that all ships can move safely and freely in the region," Hunt said.
The Revolutionary Guards said they seized the tanker at the request of Iranian maritime authorities for "not following international maritime regulations," state television reported.
On July 4, British naval forces seized an Iranian tanker in Gibraltar on suspicion of smuggling oil to Syria and breaching European Union sanctions.
Iran said it would retaliate and days later three Iranian vessels tried to block a British-owned tanker passing through the Strait of Hormuz. But on that occasion, the Iranian vessels backed off when confronted by a British Royal Navy warship.