Two British tankers have been seized in Iranian waters this evening, it is understood.
The Stena Impero and Mesdar Crude Oil Tanker are believed to be heading heading towards Iran.
Owners of the Stena Impero say they are unable to contact crew onboard.
The second ship is operated by Norbulk Shipping UK Ltd.
It is unknown how many people were aboard the Mesdar ship.
Iran has reportedly said at least one of tankers was seized after it turned off its tracker and ignored warnings.
The Ministry of Defence says it is urgently seeking information about the unfolding incident.
A meeting of the Government’s emergency committee, Cobra, is believed to be underway.
A second Cobra meeting has also been scheduled for 22:30 and will be chaired by Jeremy Hunt, according to Sky News.
The UK Chamber of Shipping has called on the government to do “whatever is necessary” to ensure the ship’s safe return.
President Donald Trump said he would talk to Britain about the tanker seizure, after a war of words earlier in the day over whether the United States had shot down an Iranian drone in the Strait of Hormuz.
He said: “We will talk to the UK and we have no written agreement but we have an agreement. They’ve been a very great ally of ours.
“So we heard about it, we heard it was one, we heard it was two, and we will be working with the UK.
“They will have a new prime minister soon, which is a good thing, and we will be working with the UK – but we have no written agreement, but I think we have an agreement which is longstanding.””
Iranian state TV has said authorities seized the Stena Impero, Reuters reports.
It is understood there are no Brits on board the Stena Impero and the crew included those of Indian, Russian, Latvian and Filipino nationalities.
According to reports in Iran, the Iranian Revolutionary Guard surrounded the tanker because it did “not following international maritime regulations”.
The vessel was shown on tracking systems taking a sudden turn in the Gulf, having been headed to Saudi Arabia.
The company that owns the vessel said it was approached by “unidentified small crafts” and a helicopter in the Strait of Hormuz.
In a statement Northern Marine Management said: “We are presently unable to contact the vessel which is now heading north towards Iran.
“There are 23 seafarers aboard. There have been no reported injuries and their safety is of primary concern to both owners and managers.”
The company said the tanker was in international waters when it was approached.
Its statement continues: “Northern Marine Management has not been able to establish contact directly with the vessel since it was notified of the incident at approximately 1600 Today, 19th July 2019.
“We are in close contact with UK government authorities.”
The Stena Impero had left Fujairah in Dubai before being intercepted in the Strait of Hormuz.
The British-operated, Liberian-flagged Mesdar, turned sharply north towards Iran’s coast on Friday afternoon after passing westward through the Strait of Hormuz into the Gulf, according to Refinitiv tracking data.
The turn took place at about 1600 GMT, the data showed, about 40 minutes after a similar course shift by the Stena Impero tanker that Iran’s Revolutionary Guards said they had captured.
There was no immediate word from the Guards about the second tanker or from the operator of the second tanker on what had prompted the change in direction along the vital international oil shipping route
A National Security Council statement said: “We are aware of reports that Iran seized a UK oil tanker. This is the second time in just over a week the UK has been targeted by [Iran].
“The US will continue to work with our allies to defend our security and interests against Iran’s malign behavior.”
An MOD spokesman said: “We are urgently seeking further information and assessing the situation following reports of an incident in the Gulf.”
Relations between Iran and the West have been increasingly strained after Britain seized an Iranian tanker in Gibraltar on suspicion of smuggling oil to Syria in breach of 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.
Refinitiv data showed the Stena Impero is a British-flagged vessel owned by Stena Bulk and it had been en route to Jubail in Saudi Arabia.
Another map tracking the location of the Stena Impero showed it making a sharp turn in the Strait of Hormuz, at the southern entrance to the Gulf, and heading toward Iranian waters.