HMS Montrose, a British frigate shadowing the tanker British Heritage, 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 British Heritage and tried to bring it to a halt as it was moving out of the Gulf into the Strait of Hormuz.

The British warship, 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,” the statement said.

“We are concerned by this action and continue to urge the Iranian authorities to de-escalate the situation in the region.”

Iran rejected the claims. Foreign minister Javad Zarif called the British allegations “worthless” in remarks to the semi-official Fars news agency.

“Apparently the British tanker has passed. What [the British] have said themselves and the claims that have been made are for creating tension and these claims have no value,” Zarif said.

Iran’s Islamic Revolutionary Guard Corp (IRGC) also issued a statement denying involvement.

US defence officials, speaking on the condition of anonymity, had earlier told Reuters that the incident happened at the northern entrance of the strait of Hormuz.

“The Royal Navy HMS Montrose, which was also there, pointed it[s] guns at the boats and warned them over radio, at which point they dispersed,” one of the officials said.

“It was harassment and an attempt to interfere with the passage,” another official said.

A spokesperson for US central command, Captain Bill Urban, said: “We are aware of the reports of Islamic Revolutionary Guard Corp Navy’s FAC/FIAC harassment and attempts to interfere with the passage of the UK-flagged merchant vessel British Heritage today near the strait of Hormuz.

“Threats to international freedom of navigation require an international solution.”