has admitted its nearest warship was one hour away from the merchant vessel
seized by Iran in the Strait of Hormuz on Friday but could do nothing to help.
UK Secretary of State for Defense Penny Mordaunt said the incident happened in Omani waters and was a "hostile act."
But she said the British Type 23 frigate HMS Montrose was 60 minutes away from being able to help the Stena Impero when it was boarded by Iranian Revolutionary Guards.
The Foreign Office summoned Iran's charge d'affaires, Mohsen Omidzamani, following the seizure of the British-flagged oil tanker.
It came as Foreign Secretary Jeremy Hunt spoke to his Iranian counterpart Mohammad Javad Zarif to express Britain's concern over the latest hike in tensions in the region.
Hunt wrote on Twitter, "Just spoke to Iranian FM Zarif and expressed extreme disappointment that having assured me last Saturday Iran wanted to de-escalate situation they have behaved in the opposite way."
"This has to be about actions not words if we are to find a way through. British shipping must and will be protected."
Zarif hit back with his own tweet, stating, "Unlike the piracy in the Strait of Gibraltar, our action in the Gulf is to uphold int'l maritime rules. As I said in NY, it is IRAN that guarantees the security of the Arabian Gulf & the Strait of Hormuz. UK must cease being an accessory to #EconomicTerrorism of the US."
Following a meeting of the government's emergency committee COBRA, a spokesman said the seizure was "a clear challenge to international freedom of navigation," adding, "As the Foreign Secretary has said, our response will be considered and robust and there will be serious consequences if the situation is not resolved."
Iran has directly linked the seizure of the tanker with Britain's role in detaining a tanker carrying Iranian oil earlier this month.
A spokesman for Iran's Guardian Council was quoted as saying "the rule of reciprocal action is well-known in international law," adding that Tehran made the right decision in the face of an "illegitimate economic war and seizure of oil tankers."
The explanation contrasts with Iran's earlier claims that the Stena Impero collided with a fishing vessel in the Arabian Gulf as tensions mount in the strategic waterway, a chokepoint for around a third of the world's sea-borne oil.
Allahmorad Afifipour, the head of Ports and Maritime Organization in southern Hormozgan province, claimed the Swedish-owned Stena Impero was in an accident with an Iranian fishing boat whose distress call it ignored.
The 30,000-tonne ship had been en route to Saudi Arabia but abruptly changed course and began sailing towards the Iranian island of Qeshm, data relayed by maritime tracking services showed. It then “went dark,” meaning its transponder was turned off at 4:29 pm UK time, and nothing has been heard from the ship or its 23 crewmembers since.
The tanker's operator, Stena Bulk, said on Friday the ship had been "in full compliance with all navigation and international regulations," but was no longer under the crew's control and could not be contacted.
Guards said it was taken to Bandar Abbas port, where the crew’s Russian, Ukrainian, Indian, Latvian and Filipino crew are being questioned.
Hunt said this morning that he was worried Iran had taken a "dangerous path."
"Yesterday's action in the Gulf shows worrying signs Iran may be choosing a dangerous path of illegal and destabilizing behavior after Gibraltar’s LEGAL detention of oil bound for Syria," Hunt said on Twitter.
"Our reaction will be considered but robust. We have been trying to find a way to resolve Grace1 issue but WILL ensure the safety of our shipping."
A Whitehall source told the Telegraph, “It looks as though the Iranian Revolutionary Guard have boarded and taken a UK-flagged ship. It appears to be linked to events around the Grace 1 tanker.”
British authorities seized the Iranian Grace 1 supertanker off the coast of Gibraltar on July 4, on suspicion it was carrying crude to Syria in violation of European Union sanctions.
The fate of the tanker has been at the center of escalating tensions between the UK and Iran and was seen as a pawn in the standoff between Tehran and the West.