Iranian boats tried to impede a British oil tanker near the Gulf - before being driven off by a Royal Navy ship, a UK government spokesman has said.
HMS Montrose positioned itself between the three boats and the tanker British Heritage before issuing verbal warnings to the Iranian vessels, he said.
He described the Iranians' actions as "contrary to international law".
Iran denied the attempted seizure. It had been threatening to retaliate for the seizure of one of its own tankers.
According to reports in US media, quoting US officials, boats believed to belong to the Iranian Revolutionary Guard Corps approached the British Heritage tanker as it was moving out of the Gulf into the Strait of Hormuz.
Guns on HMS Montrose, the British frigate escorting the tanker, were reportedly trained on the Iranian boats as they were ordered to back off. They heeded the warning and no shots were fired.
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."
Quoting the public relations office of the Islamic Revolution Guards Corps' Navy, the Guards-linked Fars news agency said, in a tweet, the IRGC "denies claims by American sources" that it tried to seize HMS Montrose.
Last week, British Royal Marines helped the authorities in Gibraltar seize an Iranian oil tanker because of evidence it was heading to Syria in breach of EU sanctions.
In response, an Iranian official said a British oil tanker should be seized if its detained ship was not released.
Iran 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 a 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 passed without incident.
The Trump administration - which has pulled out of an international agreement on Tehran's nuclear programme - has reinforced punishing sanctions against Iran.
