Britain's Government Emergency Committee is meeting now to discuss tanker incident in the Gulf.
Northern Marine said Stena Impero was also approached by a helicopter during transit of the strait of Hormuz while the vessel was in international waters.
Stena Impero is a Chemical/Oil Products Tanker built in 2018 and currently sailing under the flag of United Kingdom. Current destination of the tanker is Jubail, Saudi Arabia.
Statement from Stena Impero management company: 23 seafarers on board.
Iran's Fars News Agency reported that senior military officials rejected reports that the Liberian-flagged tanker has been seized, stressing that the Stena Impero was the only foreign ship under Iranian custody.
The incident came hours after Gibraltar's Supreme Court said it would extend by 30 days the detention of an Iranian tanker seized two weeks ago on allegations that it was heading to Syria in violation of sanctions.
Call for more security in the Gulf Further protection must be provided for merchant vessels after a British flagged tanker was seized by the Iranian Revolutionary Guards in the Strait of Hormuz, the head of the UK Chamber of Shipping said on Friday.
"We condemn unreservedly the capture of Stena Impero as she transited the Strait of Hormuz earlier today," the Chamber's chief executive, Bob Sanguinetti, said in a statement.
"This incident represents an escalation. Whilst we call for measured response, it is also clear that further protection for merchant vessels must be forthcoming to ensure enhanced security to guarantee free flow of trade in the region."
Another tanker in Gulf turned sharply towards Iran
Meanwhile, the second oil tanker, the British-operated, Liberian-flagged Mesdar, turned sharply north towards Iran's coast on Friday afternoon after passing westward through the Strait of Hormuz into the Gulf, according to Refinitiv tracking data.
The turn took place at about 1600 GMT, the data showed, about 40 minutes after a similar course shift by the Stena Impero tanker that Iran's Revolutionary Guards said they had captured.
Later, the British owner of this tanker said the ship had been temporarily boarded by armed personnel but was now free to leave.
"Communication has been re-established with the vessel and (the capitain) confirmed that the armed guards have left and the vessel is free to continue the voyage. All crew are safe and well," Norbulk Shipping UK said in a statement about the ship, the Mesdar.
US says downed drone
Tensions in the Gulf have soared in recent weeks, with Trump calling off air strikes against Iran at the last minute in June after Tehran downed an American drone, and blaming Iran for a series of tanker attacks - charges the Islamic Republic denies.
Hours before the latest incident, Iran and the US were caught in a new war of words.
Trump on Friday reiterated a claim that Washington brought down an Iranian drone that was threatening an American vessel, the USS Boxer in the Strait of Hormuz.
Iran categorically denied the allegation and deputy foreign minister Abbas Araghchi tweeted that American forces may have shot down a US drone by mistake.
Trump's National Security Advisor John Bolton said there was "no question that this was an Iranian drone".
The Revolutionary Guards released a video Friday which they said belies the US claims.
The latest escalation comes more than a year after Washington unliterally withdrew from the 2015 Iran nuclear agreement and began ratcheting up sanctions against Tehran.
On Friday Trump spoke with French leader Emmanuel Macron, the White House said, with the leaders discussing "ongoing efforts to ensure Iran does not obtain a nuclear weapon."
Earlier this month, Iran purposely overshot the deal's caps on uranium enrichment, aiming to pressure the remaining parties to make good on their promises to help prop up its economy.
Iran has repeatedly threatened to close the Strait of Hormuz if it is attacked.
The seven-minute-long footage, apparently shot from high altitude, shows a convoy of ships which the Guards said they were tracking as they passed through the Strait of Hormuz.
The vessels in the footage could not be immediately identified, although one looks similar to the USS Boxer.
Earlier a US official said Washington has "very clear evidence" that it downed the drone but gave no further details.
"If you fly too close to our ships, you're going to get shot down," the official said.
As tensions soar, Tehran's arch rival Saudi Arabia announced Friday that it would once again host US troops on its soil to boost regional security, its defence ministry said.