- Video report by ITV News Correspondent John Ray
The Royal Navy has driven off three Iranian vessels which tried to stop a commercial vessel from travelling through the Strait of Hormuz.
The altercation came a day after Iran spoke out against Britain due to the interception of a supertanker believed to be breaching EU sanctions by carrying a shipment of crude oil to Syria.
Hours later on Thursday, Gibraltar Police arrested the captain of the Iranian supertanker after authorities seized documents and electronic devices from the vessel.
Defence Secretary Penny Mordaunt said the government was "concerned" by Iran's actions and urged Tehran to "de-escalate the situation".
In a statement, she said: "The crew of HMS Montrose yesterday ensured the safe passage of the merchant vessel British Heritage through the Strait of Hormuz.
"I would like to thank the Royal Navy for their professionalism, which upheld international law and supported freedom of navigation through a shipping channel that is vital to global trade.
"The UK government is concerned by this action and we urge the Iranian authorities to de-escalate the situation."
- What can happen next as tensions between Iran and the US continue?
Conservative leadership hopeful Jeremy Hunt said Iran's attempted blockade of a British mercantile ship was a "concerning development".
He added: "I'm very proud of the Royal Navy and the role they played in keeping British asset, British shipping safe.
"We are continuing to monitor the situation very, very carefully."
John Ray said that although the Iranians are denying any involvement "it looks pretty certain that they were carrying out their threat" to retaliate for the seizure of the Iranian supertanker off Gibraltar.
It began when Donald Trump tore up that 2015 deal to rein in Iran's nuclear programme, they've reimposed sanctions which are biting in Tehran and Britain ironically really wants that deal to remain.
- ITV News Correspondent John Ray says the danger now is that either the US or Iran "miscalculate"
British oil and gas company BP, who were operating the tanker, said: “Our top priority is the safety and security of our crews and vessels. While we are not commenting on these events, we thank the Royal Navy for their support.”
A Ministry of Defence spokesman said: “HMS Montrose was forced to position herself between the Iranian vessels and British Heritage and issue verbal warnings to the Iranian vessels, which then turned away.
“We are concerned by this action and continue to urge the Iranian authorities to deescalate the situation in the region.”
On Wednesday, Iranian President Hassan Rouhani was quoted by the official IRNA news agency during a Cabinet meeting as saying last week’s seizure of the supertanker off Gibraltar was “mean and wrong”.
He warned London: “You are an initiator of insecurity and you will understand its repercussions,” without elaborating.
In the wake of the incident, former Navy chief, Lord West of Spithead, urged the UK to speed up building more frigates.
The former chief of naval staff and First Sea Lord said Britain as too few frigates, which made the UK "less secure".
He said: "It means wars are more likely and it's really important to move forward."
Lord West added that of the 13 frigates the UK has, only eight were effectively available for operations.
He said the nearest major warship to the one in the Gulf was "the other side of Suez, which I find rather worrying".
Turning to orders for new ships, Lord West said the Ministry of Defence should look as a matter of urgency at speeding up the build rate of type-26 frigates "because we have too few as a nation".
The seizure and incident in the Strait of Hormuz come at a particularly sensitive time as tensions between the US and Iran grow over the unravelling of a 2015 nuclear deal, which President Donald Trump withdrew from last year.
European parties to the nuclear deal with Iran on Tuesday said they have “deep concern” that Tehran has begun enriching uranium to a higher purity than allowed under the agreement and called for an urgent meeting of all involved in the accord.
In a joint statement by Britain, Germany, France and the European Union, the group expressed “deep concern that Iran is not meeting several of its commitments under the Joint Comprehensive Plan of Action”, or JCPOA, as the 2015 nuclear deal is known.
The US has said it will move ahead with plans to build a coalition of nations to monitor and deter Iranian threats against commercial shipping in the Persian Gulf area and in a waterway around Yemen, according to a top military officer.
Marine General Joseph Dunford, the chairman of the US Joint Chiefs of Staff, said the Pentagon has developed a specific plan, and that he believes it will be clear within a couple of weeks which nations are willing to join the effort.
Gen Dunford said he discussed the matter on Tuesday with acting secretary of defence Mark Esper and secretary of state Mike Pompeo, and that plans are coming together.
“We’re getting ready now to move out,” Gen Dunford told a small group of reporters at Fort Myer, Virginia.
“We have a pretty clear concept of what we want to do.”