The UK government has said it will provide a Royal Navy escort for British-flagged ships passing through the Strait of Hormuz - amid increasing diplomatic tensions in the Gulf.
Ship owners are being advised to give details of their route so they can be escorted by the frigate HMS Montrose.
The move follows the seizure of the British-flagged tanker Stena Impero by Iran's Revolutionary Guard on Friday.
The ship's owner said the crew are safe and co-operating with officials.
Shipping firm Stena Bulk said its 23 crew members - who are Indian, Russian, Latvian and Filipino - had been able to talk directly to their families for a limited time.
The decision to provide a military escort to merchant shipping follows a meeting of Downing Street's emergency Cobra committee to discuss the situation in the Gulf.
The latest guidance advises British-flagged vessels to inform the Department of Transport if they are passing through the Strait of Hormuz so that a Royal Navy warship can provide a military escort.
"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," a government spokesman said.
"Freedom of navigation is crucial for the global trading system and world economy, and we will do all we can to defend it," he added.
"This move will provide some much needed safety and reassurance to our shipping community in this uncertain time. However, we will continue to push for a de-escalation of tensions in the region and the safe return of our seafarers."
The UK Chamber of Shipping said it welcomed the government's decision, calling the announcement "an encouraging step".
UK Prime Minister Boris Johnson's spokesman added that the government was focused on securing the release of the detained Stena Impero tanker and its crew, while "de-escalating current tensions" with Iran.
Stena Bulk has said it is discussing the release of the tanker and has asked "local authorities" for access to the vessel.
On Wednesday night, the MoD said HMS Montrose escorted two merchant vessels travelling together through the strait.
