Claims Iranian vessels tried to stop British tanker from travelling through the Strait of Hormuz
The Royal Navy said it has driven off three Iranian vessels which tried to stop a commercial tanker from travelling through the Strait of Hormuz.
The Iranian Revolutionary Guard denied the allegations, saying if it had received orders to seize any ships it would have done so immediately.
The reported 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.
"Contrary to international law, three Iranian vessels attempted to impede the passage of a commercial vessel, British Heritage, through the Strait of Hormuz," Britain's Ministry of Defence 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."
Downing Street said the security situation in the Gulf is being closely monitored.
The prime minister'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."
In a statement carried by the semi-official Fars news agency, the Revolutionary Guard's navy insisted there had been no clashes with foreign ships, "especially British boats".
Foreign minister Mohammad Javad Zarif also dismissed the British account as "worthless", saying the claims were being made to "create tension".
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.
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 US 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 EU, 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."
British Heritage is a BP Century class Suezmax tanker, the largest to be able to travel through the Suez Canal and capable of carrying more than one million barrels of oil, according to the BP Shipping website.
Registered at the port of Douglas, in the Isle of Man, the ship is operated by BP usually with a crew of around 25 people.
A spokesman for BP, which operates the British Heritage 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."
It is understood the ship was not carrying cargo.
HMS Montrose has also been in the region since April as part of a three-year deployment supporting counter-terror and anti-smuggling work.
The Royal Navy has had a continuous presence in the region for more than 30 years following the Iran-Iraq war in 1980 in what is known as Operation Kipion.