Britain has denounced the Iranian seizure of a British-flagged oil tanker in the Gulf as a “hostile act”, rejecting Tehran’s explanation that it had seized the vessel because it had been involved in an accident.

Iran’s Revolutionary Guards posted a video online showing speedboats pulling up alongside the Stena Impero tanker, its name clearly visible.

Troops wearing ski masks and carrying machine guns rappelled to its deck from a helicopter, the same tactics used by British Royal Marines to seize an Iranian tanker off the coast of Gibraltar two weeks ago.

Friday’s action in the global oil trade’s most important waterway has been viewed in the West as a major escalation after three months of confrontation that has already taken Iran and the US to the brink of war.

It follows weeks of threats from Tehran to retaliate for Britain’s seizure of the Iranian tanker Grace 1, accused of violating sanctions on Syria.

British Defence Secretary Penny Mordaunt on Saturday called the incident a “hostile act”.

Foreign Secretary Jeremy Hunt said he had expressed “extreme disappointment” by phone to his Iranian counterpart, Mohammad Javad Zarif.

Britain also summoned the Iranian charge d’affaires in London.

A spokesman for Iran’s Revolutionary Guards, Brigadier-General Ramezan Sharif, said Tehran had seized the ship in the Strait of Hormuz despite the “resistance and interference” of a British warship which had been escorting it. No British warship was visible in the video posted by the Guards.

Iran’s Fars news agency said the Guards had taken control of the Stena Impero on Friday after it collided with an Iranian fishing boat whose distress call it ignored.

The vessel, carrying no cargo, was taken to the Iranian port of Bandar Abbas. It would remain there with its 23 crew, 18 of them Indians, while the accident is investigated, Iranian news agencies quoted the head of Ports and Maritime Organisation in southern Hormozgan province, Allahmorad Afifipour, as saying.

Mr Zarif told Britain’s Mr Hunt that the ship must go through a legal process before it could be released, Iran’s ISNA news agency reported.

The US, which tightened sanctions against Iran in May with the aim of halting its oil exports altogether, has been warning for months of an Iranian threat to shipping in the strait.

Another oil tanker, the Mesdar, was also boarded by Iranian personnel on Friday and temporarily forced to divert towards Iran, but later was allowed to continue on its route through the strait.

On Saturday, Algeria’s APS news agency said the Mesdar was owned by the country’s state oil company Sonatrach.

France, Germany and the European Union joined Britain in condemning the seizure.

The three big European countries are signatories to a 2015 nuclear deal between Tehran and world powers that Washington undermined by quitting last year, setting Iran’s already fragile relations with the West on a downward spiral.

“Just spoke to … Zarif and expressed extreme disappointment that having assured me last Saturday Iran wanted to de-escalate situation, they have behaved in the opposite way,” tweeted British Foreign Secretary Hunt.

“This has to be about actions not words if we are to find a way through.”

Earlier he said London’s reaction would be “considered but robust”, and it would ensure the safety of its shipping.

The past three months of escalation have seen the US and Iran come as close as ever to direct armed conflict.

The US has blamed Iran for a series of attacks on shipping around the Strait of Hormuz. Tehran rejects the allegations.

The US is sending military personnel and resources to Saudi Arabia for the first time since the US invasion of Iraq in 2003.