Britain has ordered its navy to escort UK-flagged ships through the Strait of Hormuz after Iranian soldiers seized a tanker in the flashpoint entrance to the Gulf.
"The Royal Navy has been tasked to accompany British-flagged ships through the Strait of Hormuz, either individually or in groups, should sufficient notice be given of their passage," the defence ministry said in a statement to AFP.
The UK has deployed one of its Type 45 warships to help escort commercial ships through the Strait of Hormuz after Iran seized a British vessel amid heightening tensions with the West.
The frigate, HMS Duncan, will operate alongside the Royal Navy's HMS Montrose Type 23 to shepherd British-flagged ships through the strait.
The week-long standoff over the British-flagged Stena Impero and its 23-member crew has inflamed tensions between the Islamic republic and the United Kingdom.
Britain responded on Monday by proposing a European-led mission that could secure the passage of vessels through the world's busiest oil shipping lane.
But France said Thursday it was not willing to send extra military assets to the Gulf, but would share information and coordinate its currently deployed assets.
Iran had earlier warned Britain that it intended to retaliate for UK marines' involvement of the seizure of its own supertanker near Gibraltar on July 4.
Britain currently has the HMS Montrose warship in the Gulf and a handful of smaller naval vessels.
The Montrose had tried to rush in to rescue the Stena but arrived too late to the scene.
Britain has already raised its security level in the region to the highest level and advised all boats in Iranian waters not to enter the strait.
Its guidance before Thursday was for ships to notify the navy and receive instructions on "the safest way to transit" into the Gulf.
"It is not possible for the Royal Navy to provide escorts for every single ship," now-former foreign secretary Jeremy Hunt told parliament on Monday.
The UK department of transport had earlier advised British-registered ships not to sail through the area.
Hunt told parliament that two to three UK-flagged ships pass through the strait daily.
He added that the Montrose had escorted 30 merchant vessels through the strait in 17 separate transits as of Monday.
The 33-kilometre (22-mile) wide passageway provides the eastern entrance and exit point into the Gulf and runs between the United Arab Emirates and Iran.