Iran's state news agency IRNA quoted a military source as saying the vessel had turned off its tracker, ignored warnings from the Revolutionary Guards and was sailing in the wrong direction in a shipping lane.
But Iran seems to view the armed takeover of the Stena Impero as a carefully calibrated response to the July 4 taking of an Iranian supertanker off the coast of Gibraltar, an operation in which Britain's Royal Marines played a major role.
British Defense Secretary Penny Mordaunt told Sky News the takeover was a "hostile act" by Iran.
"I will shortly attend a COBR [national security] meeting to review what we know and what we can do to swiftly secure the release of the two vessels - a British-flagged vessel and a Liberian-flagged vessel", he said. Britain also summoned the Iranian charge d'affaires in London.
Iran's Revolutionary Guard (IRGC) has released a video showing the moment its operatives captured the UK-flagged tanker Stena Impero. After a short standoff, a British warship drove them away.
Iran had threatened seizing a British tanker since Gibraltar detained an Iranian super-tanker in early July for carrying oil to Syria in violation of European Union sanctions. It showed the vessel's destination as the Saudi port of Jubail on the Gulf.
Earlier, Fars News Agency reported that all 23 crew members on the detained tanker were now at the port of Bandar Abbas and will remain aboard the vessel until the end of the investigation.
He said that Iran moves to "confront the illegitimate economic war and seizure of oil tankers is an instance of this rule and is based on global rights".
Current tensions have been escalating since President Donald Trump withdrew the US from Iran's 2015 nuclear deal with world powers and imposed economic sanctions on Iran, including its oil exports.
Undated photograph shows the Mesdar, a British-operated oil tanker in Fawley, Britain obtained by Reuters on July 19, 2019.
The 2015 accord, of which Britain was a signatory, was created to prevent Iran from developing nuclear weapons in return for a lifting of sanctions.
The European countries opposed the Trump administration's decision to abandon the agreement past year, but have so far failed to fulfill promises to Iran of providing alternative means for it to access world trade.
British Foreign Secretary Jeremy Hunt also warned that there will be "serious consequences" if the situation is not resolved.
"Our action in the Persian Gulf is to uphold int'l rules", he wrote in a tweet on Saturday, calling on Britain to "cease being an accessory" to USA sanctions against Iran.
In recent weeks, the U.S. and its allies have found themselves responding in a tit-for-tat with Iran.
"That's the type of concept we're trying to envision, DOD, throughout the Strait so we don't get into a military fight", he said.
British ministers are expected to face tough questions about the decision to seize an Iranian tanker two weeks ago, without ensuring that it could protect British-owned shipping in the strait of Hormuz. "Escalation risks a slide into an even deeper conflict". Tensions in the Gulf have soared in recent weeks, with US President Donald Trump in June calling off air strikes against Iran at the last minute after Tehran downed a US drone, and blaming the country for a series of tanker attacks.
Both Manila and New Delhi said they were in touch with Tehran to seek the release of their nationals.
"We are aware of reports that Iranian forces seized a British oil tanker", spokesman Garrett Marquis said.
The operator said communication had since been re-established with the ship and the crew were unharmed.
The vessel had been heading to a port in Saudi Arabia and suddenly changed course after passing through the strait.