The British navy said it prevented three Iranian paramilitary vessels from impeding the passage of a British oil tanker in the Strait of Hormuz early Thursday.
The move came a day after Iran's president warned of repercussions for the seizure of its own supertanker.
The tensions in the Middle East has been among the factors sending oil prices higher.
Iran's Revolutionary Guard denied the allegations, saying if it had received orders to seize any ships it would have done so immediately.
The incident came at a time of heightened tensions over Iran's unraveling nuclear agreement with world powers.
Iran recently began breaching uranium enrichment limits in response to the Trump administration's withdrawal from the agreement last year and its imposition of sweeping sanctions.
In recent months, the U.S. has dispatched thousands of additional troops, an aircraft carrier, bombers and advanced fighter jets to the region.
Washington has blamed Iran for a series of mysterious attacks on oil tankers — charges denied by Tehran — and said it shot down an American drone in international airspace. Iran said it downed the drone after it veered into its own airspace.
The U.K. government said in a statement that British navy vessel HMS Montrose had been accompanying the commercial ship, British Heritage, through the narrow Strait of Hormuz, a crucial waterway for energy shipments .
"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 government statement said.
"We are concerned by this action and continue to urge the Iranian authorities to de-escalate the situation in the region," it added.
The HMS Montrose is currently on a three-year mission at the British navy's support facility in Bahrain, the hub of Britain's naval operations east of the Suez Canal.
The U.S. 5th Fleet in Bahrain and Central Command declined to comment on the incident.
Maritime security risk firm Dryad Global described the British Heritage as an oil tanker operated by BP and registered in the Isle of Man.
Lloyd's List, a publication specializing in maritime affairs, said Shell had chartered the ship from BP.
The Associated Press contributed to this article.