Authorities impounded the Stena Impero with 23 crew members aboard off the port of Bandar Abbas after the Islamic Revolutionary Guard Corps seized it Friday in the highly sensitive Strait of Hormuz.
Video footage released by the Guards showed a ship with the oil tanker's markings being surrounded by speedboats, before troops in balaclavas descend a rope from a helicopter onto the vessel.
In an audio recording of a radio exchange, an Iranian officer can be heard telling the tanker to change course.
"You are ordered: change your course to three six zero... immediately. If you obey, you will be safe," he said.
The British frigate HMS Montrose intervenes to inform the Stena its "passage must not be impaired, impeded, obstructed or hampered" under international law.
The Iranians then tell the British warship: "Foxtrot 236 this is Sepah navy patrol boat. No challenge is intended... I want to inspect the ship for security reason."
The authenticity of the recording, obtained and released by London-based maritime security risk analysts Dryad Global, was confirmed by the UK defence ministry.
London has warned its ships to avoid the Strait of Hormuz, a chokepoint for about a third of the world's sea-borne oil.
On Sunday evening, Iran's English-language Press TV broadcast live footage from the deck of the seized ship, flying an Iranian flag.
"IRGC forces manage to lead tanker to Iran shores despite UK warship's interference," said a news ticker on the channel.
- Iran urges crew's 'cooperation' -
Iran opened the probe after detaining the ship on allegations it failed to respond to distress calls and turned off its transponder after hitting a fishing boat.
Its crew is made up of 18 Indians, including the captain, three Russians, a Latvian and a Filipino.
"All of them are in full health... anchored in a safe place," said Allah-Morad Afifipoor, director-general of the Hormozgan province port and maritime authority.
"The investigation depends on the cooperation by the crew members on the vessel," he told Press TV.
The ship's owner said it was in "international waters" when it was "attacked by unidentified small crafts and a helicopter".
Stena Bulk's chief Erik Hanell said Sunday the firm had formally asked Iranian authorities for permission to visit the vessel's crew, and was waiting for a response.
Tehran has been at loggerheads with Washington since May 2018, when President Donald Trump unilaterally withdrew the US from a landmark 2015 deal putting curbs on Iran's nuclear programme in exchange for sanctions relief.
Tensions in the Gulf have escalated since May this year, when the US boosted its military presence in the region in response to "indications of a credible threat by Iranian regime forces".
The US administration reimposed tough sanctions on Iran, which retaliated by increasing its enrichment of uranium beyond limits set in the nuclear accord.
Trump called off air strikes against Iran at the last minute in June after the Islamic republic downed a US drone, one of a string of incidents including attacks on tankers in the Gulf.
- 'Tit-for-tat' -
Britain summoned Iran's charge d'affaires on Saturday and urged his country to de-escalate tensions and release the tanker.
Foreign Secretary Jeremy Hunt said the seizure showed "worrying signs Iran may be choosing a dangerous path of illegal and destabilising behaviour".
Hunt called it a "tit-for-tat" situation, which flared hours after a Gibraltar court extended by 30 days the detention of an Iranian tanker seized two weeks ago on allegations of breaching UN sanctions against Syria.
On Sunday junior defence minister Tobias Ellwood told Sky News that Britain was "going to be looking at a series of options", without giving further details.
Hunt has said parliament will be updated on Monday about what further measures the British government would take.
Iran has remained defiant.
Foreign Minister Mohammad Javad Zarif tweeted that Trump's national security adviser John Bolton "is turning his venom against the UK in hopes of dragging it into a quagmire".
Nearby Oman, which maintains strong ties with Iran, joined calls for the release of the Stena Impero and urged London and Tehran to resolve the dispute.
Separately, Iran said one of its tankers held in its regional rival Saudi Arabia since being forced to seek repairs in the kingdom is returning home.
The Happiness 1 "has been released following negotiations and is now moving toward Persian Gulf waters," said transport minister Mohammad Eslami, quoted by state news agency IRNA.