Britain said it was urgently seeking information about the Stena Impero after the tanker, which had been heading to a port in Saudi Arabia, suddenly changed course after passing through the Strait of Hormuz at the mouth of the Gulf.
A second tanker, the British-operated Mesdar, also made a sharp change of direction in the Gulf around the same time.
Britain's foreign minister said a second vessel had been seized and described the capture of both tankers as unacceptable. Iran's Guards made no mention of another ship.
Later Friday, Iran's Revolutionary Guards said they have not captured the British-operated, Liberian-flagged ship Mesdar in the Gulf, Iran's semi-official Tasnim news agency reported, quoting regional military sources.
"Despite reports, the ship has not been seized ... and was allowed to continue its course after being warned about safety issues by Iranian forces," the report said.
Relations between Iran and the West have become increasingly tense since British naval forces seized an Iranian tanker in Gibraltar on July 4 on suspicion of smuggling oil to Syria in breach of European Union sanctions.
U.S. President Donald Trump said he would talk to Britain about the tanker seizure, after a war of words earlier in the day over whether the United States had shot down an Iranian drone in the Strait of Hormuz.
Oil prices gained on Friday, trading above $62 a barrel, after the latest spike in tensions along a waterway through which a fifth of the world's crude supplies pass.
The Guards, an elite force under the command of Supreme Leader Ayatollah Ali Khamenei, said they seized the Stena Impero at the request of Iranian authorities for "not following international maritime regulations," state television reported.
Northern Marine Management said the Stena Impero was heading north towards Iran. Northern Marine Group is owned by Stena AB.
Refinitiv tracking data showed the British-operated and Liberian-flagged Mesdar also changed course sharply after entering the Gulf and started heading towards the Iranian coast, about 40 minutes after the Stena Impero shifted direction.
British foreign minister Jeremy Hunt said "I'm extremely concerned" about the seizures of both ships.
There was no immediate word from the Guards about the second tanker. Mesdarâ€™s manager Norbulk said it was checking the report.
Iran had said it would retaliate against the seizure in Gibraltar and days later three Iranian vessels tried to block a British-owned tanker passing through the Strait of Hormuz. The Iranian ships backed off when confronted by a British navy ship.
Changing Course"We are urgently seeking further information and assessing the situation following reports of an incident in the Gulf," a spokesman for Britain's Ministry of Defence said after the Stena Impero changed course.
A person familiar with the matter said the British government's emergency committee was meeting over the issue.
Refinitiv data showed the Stena Impero is a British-flagged vessel owned by Stena Bulk. It showed the vessel's destination as the Saudi port of Jubail on the Gulf.
The map tracking the ship showed it veering off course with a sharp turn north at about 1517 GMT and heading toward the Iranian coast.
The Mesdar made its shift toward Iran at about 1600 GMT.
"We received reports that the British Stena Impero oil tanker was causing incidents and, therefore, we asked the military to direct it to Bandar Abbas port for the necessary probes," Allahmorad Afifipour, head of Hormozgan's maritime authority, told Iran's semi-official news agency Tasnim.
Tasnim also quoted unnamed sources saying tanker had been causing pollution by dumping oil residue.
An Iranian military source said the Stena Impero "had turned off its tracker and ignored several warnings by the Guards before being captured," the official IRNA news agency reported.
State television said the tanker was taken to a coastal area and turned over to the authorities to take the necessary legal steps, the television added.
"Piracy"Supreme Leader Khamenei said on Tuesday Iran would respond to Britainâ€™s "piracy" over the seizure of the Iranian oil tanker in Gibraltar.
Iran has repeatedly called for the ship's release, denies the allegation that the tanker was taking oil to Syria in violation of sanctions and says Gibraltar and Britain seized the vessel on the orders of Washington.
Guy Platten, secretary general of the International Chamber of Shipping, called on all those involved in Friday's shipping incident to seek a swift resolution to the incident that threatened the safety of seafarers.
"Freedom of navigation is vital for global trade and we encourage all nations to uphold this fundamental principle of maritime law," he said.