The UK's Foreign Secretary Jeremy Hunt has said he is "extremely concerned" by the "unacceptable" seizure of two oil tankers by Iranian authorities in the Strait of Hormuz.
Mr Hunt confirmed that he will attend a meeting of the British government's emergency committee Cobra tonight amid the escalation of tensions.
The Stena Impero, which is registered in the UK, was seized by the Iranian Revolutionary Guard in the Strait of Hormuz for "violating international maritime rules", said Iran's Revolutionary Guards.
And a second oil tanker, the Liberia-flagged Mesdar, which is owned and operated by Glasgow-based firm Norbulk, appeared to veer off course towards the Iranian coast, according to its path on the Marine Traffic website.
The Mesdar's operator said communication had since been re-established with the ship after it was boarded by armed guards at about 5.30pm today.
In a statement they said: "Communication has been re-established with the vessel and master confirmed that the armed guards have left and the vessel is free to continue the voyage. All crew are safe and well."
Mr Hunt said: "Their crews comprise a range of nationalities, but we understand there are no British citizens on board either ship.
"Our ambassador in Tehran is in contact with the Iranian Ministry of Foreign Affairs to resolve the situation and we are working closely with international partners.
"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."
Earlier, Iran's Revolutionary Guards announced that they had confiscated a British tanker in the strategic Strait of Hormuz for breaking "international maritime rules".
The Stena Impero tanker "was confiscated by the Revolutionary Guards at the request of Hormozgan Ports and Maritime Organisation when passing through the strait of Hormuz, for failing to respect international maritime rules," the Guards' official website Sepah news announced.
A statement from Stena Bulk said the firm had lost contact with the crew of 23 after it was approached by "unidentified small crafts and a helicopter" at around 4pm.
The statement said: "Stena Bulk and Northern Marine Management can confirm that at approximately 1600 BST on 19th July UK registered vessel Stena Impero (built 2018, 49,683 DWT) was approached by unidentified small crafts and a helicopter during transit of the Strait of Hormuz while the vessel was in international waters.
"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 priority of both vessel owner Stena Bulk and ship manager Northern Marine Management is the safety and welfare of the crew."
Richard Dalton, former British ambassador to Iran, suggested the owners of the Stena Impero had been "rash" in sailing the tanker through the Strait of Hormuz.
Speaking to Sky News, he said Iran's supreme leader Ayatollah Ali Khamenei had promised retaliation following the detention of Iran's Grace 1 tanker in Gibraltar.
Mr Dalton said: "With hindsight, it's easy to say that this was a somewhat reckless act by the owners, given that there was no British naval vessel in the vicinity."
He said the Iranians had "lost their cool" despite recent "constructive discussions" over the Grace 1.
"I don't think the Iranians will continue to try to seize British vessels given they have got what they want, which is something to hold in a negotiation with Britain about their cargo held, they consider illegally, in Gibraltar."
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.
"We are urgently seeking further information and assessing the situation following reports of an incident in the Gulf," a spokesman for Britain's Ministry of Defence has said.
I understand an official-level Cobra meeting is looking into this. Govt source: "It's the kind of thing the Iranians would do." https://t.co/I8Uu7MysQJ— Tim Shipman (@ShippersUnbound) July 19, 2019
Prime Minister Theresa May's office declined to comment.
Additional reporting PA