The British Heritage was crossing into the Strait of Hormuz area when the Iranian vessels approached it, demanding the tanker change course and stop in nearby Iranian territorial waters
Three Iranian ships attempted to "impede the passage" of a British oil tanker in Gulf waters, forcing a UK frigate to intervene, the British government said Thursday.
"Contrary to international law, three Iranian vessels attempted to impede the passage of a commercial vessel, British Heritage, through the Strait of Hormuz," a UK government statement said of the incident, which occurred on Wednesday.
"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's President Hassan Rouhani on Wednesday warned Britain of unspecified "consequences" over the detention of one of the country's oil tankers off Gibraltar.
Iran condemned the "illegal interception" of its Grace 1 tanker, which was halted in Gibraltar last week with the aid of British Royal Marines.
Officials in Gibraltar - a British overseas territory on Spain's southern tip - said the cargo was believed to be destined for Syria, which is subject to European sanctions.
Further heightening tensions, US President Donald Trump on Wednesday warned Tehran that sanctions would soon be "increased substantially" on Iran over its decision to enrich uranium beyond the limit imposed by a 2015 nuclear deal that Washington withdrew from last year.