A Royal Navy warship has driven off three Iranian boats which tried to stop a British oil tanker in the Strait of Hormuz, the Ministry of Defence (MOD) has said.
An MOD spokesman said HMS Montrose was "forced to position herself" between the three boats and commercial ship British Heritage before issuing verbal warnings to the Iranian vessels.
He said the Iranian boats "attempted to impede" British Heritage in actions that were "contrary to international law".
The spokesman said the MOD was "concerned" by the incident and called on Iran to "de-escalate the situation in the region".
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.
It is believed the boats belonged to the Iranian Revolutionary Guard Corps.
Iran has denied any involvement in the incident.
American officials, quoted by the US media, said HMS Montrose "pointed its guns at the boats and warned them over the radio".
HMS Montrose had been shadowing another British tanker, the Pacific Voyager, as it travelled through the Strait of Hormuz.
The incident comes a day after Iran warned that Britain will face "repercussions" following the seizure of an Iranian supertanker by Royal Marines near Gibraltar.
It was believed the ship was carrying oil to Syria, in breach of European Union sanctions.
Iran has denied this and said the tanker was travelling "somewhere else".
Tensions have been rising in the region since the US pulled out of the 2015 nuclear deal with Iran and reimposed sanctions.
Since then, the US has accused Iran of being behind the attacks on six oil tankers in the region, as well as the downing of an American drone.
A top US general has since called for an international military coalition to safeguard the Gulf's waters.