Iranian boats tried to impede a British oil tanker near the Gulf – before being driven off by a Royal Navy ship, the Ministry of Defence has said.
HMS Montrose, a British frigate shadowing the BP-owned tanker, was forced to move between the three boats and the tanker, a spokesman said.
He described the Iranians’ actions as “contrary to international law”.
Iran had threatened to retaliate for the seizure of one of its own tankers, but denied any attempted seizure.
Boats believed to belong to Iran’s Islamic Revolution Guard Corps (IRGC) approached tanker British Heritage and tried to bring it to a halt as it was moving out of the Gulf into the Strait of Hormuz.
Guns on HMS Montrose were trained on the Iranian boats as they were ordered to back off, US media reported. The boats heeded the warning and no shots were fired.
The BBC has been told British Heritage was near the island of Abu Musa when it was approached by the Iranian boats.
HMS Montrose had been shadowing British Heritage from a distance but came to its aid once the Iranian boats began harassing the tanker, BBC defence correspondent Jonathan Beale said.
Although Abu Musa is in disputed territorial waters, HMS Montrose remained in international waters throughout.
A UK government spokesman said: “Contrary to international law, three Iranian vessels attempted to impede the passage of a commercial vessel, British Heritage, through the Strait of Hormuz.
“We are concerned by this action and continue to urge the Iranian authorities to de-escalate the situation in the region.”
Prime Minister Theresa May’s official spokesman said: “We have long-standing maritime presence in the Gulf.
“We are continuously monitoring the security situation there and are committed to maintaining freedom of navigation in accordance with international law.”
The navy of Iran’s Islamic Revolution Guard Corps has denied claims it tried to seize the tanker, Iranian news agencies said. The navy added there had been no confrontation with any foreign vessels in the past 24 hours.
Iranian foreign minister Mohammad Javad Zarif said the UK made the claims “for creating tension”.
“These claims have no value,” Mr Zarif added, according to the Fars news agency.
The relationship between the UK and Iran has become increasingly strained, after Britain said the Iranian regime was “almost certainly” responsible for the attacks on two oil tankers in June.
Last week, British Royal Marines helped the authorities in Gibraltar seize an oil tanker because of evidence it was carrying Iranian crude oil to Syria in breach of EU sanctions.
The Government of Gibraltar said it would not comment on matters relating to the vessel as it was the subject of a police investigation. It said the matter was also now in the Supreme Court.
The Port of Gibraltar’s live map showed Grace 1 remained anchored about 3km off the east coast of Gibraltar on Thursday.
An Iranian official said a British oil tanker should be seized if Grace 1 was not released.
ran also summoned the British ambassador in Tehran to complain about what it said was a “form of piracy”.
On Wednesday, Iranian President Hassan Rouhani mocked the UK, calling it “scared” and “hopeless” for using Royal Navy warships to shadow another British tanker in the Gulf.
HMS Montrose had shadowed British tanker the Pacific Voyager for some of the way through the Strait of Hormuz, but that journey had passed without incident.
“You, Britain, are the initiator of insecurity and you will realise the consequences later,” Mr Rouhani said.
The Royal Navy has a frigate, four minehunters and a Royal Fleet Auxiliary support ship already stationed in a permanent Naval Support Facility in the region, at Mina Salman in Bahrain.
This is enough to provide reassurance, but probably not to deal with a crisis, the BBC’s Jonathan Beale said.
“HMS Montrose will not be able to provide protection for every commercial vessel in the Gulf with links to the UK,” he added.
“Ministers will now have to contemplate sending another Royal Navy warship to the region. But in doing so, that may only further escalate tensions with Iran, which is something the government wants to avoid.”
International Trade Secretary Liam Fox thanked the crew of HMS Montrose, adding: “It is our duty as a parliament to ensure that all those forces are adequately resourced.”
The UK has also been pressing Iran to release British-Iranian mother Nazanin Zaghari-Ratcliffe who was jailed for five years in 2016 after being convicted for spying, which she denies.