A British-flagged oil tanker has been seized in the Gulf by the Iranian Revolutionary Guard, Iran media say.
The owners of the Stena Impero, which was bound for Saudi Arabia, say they have been unable to contact the vessel and it is “heading north towards Iran”.
They say there are 23 personnel on board and that it was approached by “unidentified small crafts and a helicopter” in the Strait of Hormuz.
The Foreign Office is “urgently” looking into the reports.
The government’s emergency committee, Cobra, is meeting in Whitehall to discuss the incident.
A maritime tracking website is also reporting a second tanker with UK links has made a sharp turn towards Iran.
The Ministry of Defence said it was aware of reports about a second vessel – the Liberian MV Mesdar is British owned.
These latest developments come amid heightened tensions between the UK and Iran.
The Tasnim news agency quoted the Ports and Maritime Organisation of Iran as saying: “We received some reports on the British oil tanker, Stena Impero, causing problems.
“We asked the military forces to guide this tanker towards Bandar Abbas port to have the required investigations carried out.”
The Iranian Revolutionary Guard-affiliated news agency said the tanker was seized for breaking three regulations: shutting down its GPS; going through the exit of the Strait of Hormuz rather than the entrance; and ignoring warnings.
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Stena Bulk, the vessel owner, and Clydebank-based ship manager Northern Marine Management confirmed the UK-registered Stena Impero was approached at around 16:00 BST on Friday while it was in international waters.
A statement said: “We are presently unable to contact the vessel which is now heading north towards Iran.
“There have been no reported injuries and their safety is of primary concern to both owners and managers.”
A Foreign Office statement said: “We are urgently seeking further information and assessing the situation following reports of an incident in the Gulf.”
Tensions between the UK and Iran flared up earlier this month when Royal Marines seized an Iranian tanker which was suspected of breaking EU sanctions.
The UK suspected Grace 1, detained on 4 July near Gibraltar, was carrying oil bound for Syria.
In response to the seizure, Iran threatened to seize a British oil tanker.
On 9 July, the UK raised the threat to British shipping in Iranian waters in the Gulf to “critical” – the highest level.
A day later, Iranian boats attempted to impede a British oil tanker in the region, before being warned off by a Royal Navy ship, according to the Ministry of Defence.
Iran denied any attempted seizure.
Labour deputy leader Tom Watson tweeted that the latest reports were of “real concern”, adding that “any move to seize a British tanker would be a significant and harmful escalation of a situation where de-escalation is needed”.
Sir Richard Dalton, former UK ambassador to Iran, told the BBC that Iran was “trying to put a scare into the owners and operators of tankers” in the region.
US President Donald Trump said he would talk to the UK following the claims that Iran had seized a British-registered tanker.
A White House National Security Council spokesman said it was the second time in just over a week the UK had been “the target of escalatory violence” by Iran.
He added: “The US will continue to work with our allies and partners to defend our security and interests against Iran’s malign behaviour.”
Iran has been blamed by the US for attacks on tankers which have happened in the world’s key shipping area since May. Tehran denies all the accusations.