Britain, however, said Iran had seized two ships in the Gulf, incidents Foreign Secretary Jeremy Hunt condemned as 'unacceptable'.
Iran's Revolutionary Guards said Friday they had confiscated a British tanker in the Strait of Hormuz for breaking "international maritime rules" as tensions mount in the highly sensitive waterway.
Britain, however, said Iran had seized two ships in the Gulf, incidents Foreign Secretary Jeremy Hunt condemned as "unacceptable".
The British owner of one of the tankers, the Liberian-flagged Mesdar, said the ship had been temporarily boarded by armed personnel but was now free to leave.
The latest incident came as US President Donald Trump insisted on Friday that the military had downed an Iranian drone that was threatening a US naval vessel in the Strait of Hormuz, despite denials from Tehran.
The Stena Impero tanker "was confiscated by the Revolutionary Guards at the request of Hormozgan Ports and Maritime Organisation when passing through the Strait of Hormuz, for failing to respect international maritime rules," the Guards' official website Sepahnews said.
The UK-flagged tanker "was led to the shore and handed over to the organisation to go through the legal procedure and required investigations," it said.
Tanker tracking service Marine Traffic showed that the Swedish-owned Stena Impero last signalled its location near the Island of Larak in the highly sensitive waterway at 9:00 PM local time (1630 GMT).
The Stena Impero's Swedish owner said the ship was transiting the Strait of Hormuz and in "international waters" when it was "attacked by unidentified small crafts and a helicopter".
"We are presently unable to contact the vessel which is now tracking as heading north towards Iran," a statement said.
Hunt said he was "extremely concerned by the seizure of two naval vessels by Iranian authorities in the Strait of Hormuz" -- through which nearly a third of the world's oil is transported.
"These seizures are unacceptable," said Hunt.
Britain confirmed that one of the boats seized was British-registered.
Watch: Iran releases footage as 'proof' disproving the US downed drone claim
The other was the Liberian-flagged Mesdar, but its owner, British company Norbulk Shipping, said that communication had been re-established and it was now free to continue its voyage.
"All crew are safe and well," it said in a statement.
The company said the Mesdar was boarded by armed personnel at around 1630 GMT, but the armed personnel had left by 2000 GMT.
Iran's Fars News Agency reported that senior military officials rejected the 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.
British authorities' detention of the Grace 1 supertanker sparked outrage in Tehran, with the supreme leader, Ayatollah Ali Khamenei accused London of "piracy" and vowing retaliation.
US says downed drone
Tensions in the Gulf have soared in recent weeks, with Trump calling off airstrikes 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 tanker incident, Iran and the US were caught in a new war of words.
Trump said on Thursday that Washington brought down an Iranian drone that was threatening the American vessel, the USS Boxer in the Strait of Hormuz. On Friday he reiterated his claim.
Iran categorically denied the allegation and deputy foreign minister Abbas Araghchi tweeted that American forces may have shot down a US drone by mistake.
"No doubt about it, no -- we shot it down," Trump said on Friday, and he warned Iran against doing anything "foolish," saying it would pay a heavy price if it did so.
Trump's National Security Advisor John Bolton said there was "no question that this was an Iranian drone".
The Revolutionary Guards released a video on Friday which they said belies the US claims.
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.
"The US will continue to work with our allies and partners to defend our security and interests against Iran's malign behaviour," National Security Council spokesman Garrett Marquis said in a statement, denouncing "escalatory violence by the Iranian regime."
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.
Trump spoke Friday with French leader Emmanuel Macron on the ongoing situation surrounding Iran, the White House said.
The two leaders "discussed ongoing efforts to ensure Iran does not obtain a nuclear weapon," the White House said in a brief readout of the call.
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.