UK warships will accompany all British-flagged vessels through the Strait of Hormuz in a change of policy after the government earlier said it did not have the resources to protect every ship from Iranian aggression.
A British government spokesman said ships passing through the strategically important chokepoint would be accompanied either individually or in a convoy.
The frigate HMS Montrose carried out the first such mission on Wednesday night into Thursday, shipping industry sources told Sky News.
"The Royal Navy has been tasked to accompany British-flagged ships through the Strait of Hormuz, either individually or in groups, should sufficient notice be given of their passage," a British government spokesperson said.
“Freedom of navigation is crucial for the global trading system and world economy, and we will do all we can to defend it.”
Tensions between Tehran and London have worsened since the Iranian Revolutionary Guard Corps seized the Stena Impero, a Swedish-owned tanker sailing under the British flag, in the Strait of Hormuz on Friday.
Iran said the move was in response to British forces capturing an Iranian oil tanker near Gibraltar that was said to be carrying crude oil to Syria in breach of EU sanctions.
After the seizure of Stena Impero, Britain fast-tracked the arrival of a second warship to the region. HMS Duncan, a Type 45 destroyer, will arrive in the Gulf next week.
Britain has called for a European-led maritime protection mission to ensure safe shipping through the Strait of Hormuz, which sees a fifth of the world’s oil passing through.
On Wednesday, Sweden revealed it was talking to Britain and Iran about reducing tensions as Tehran hinted that it may be keen to do a ship swap.
“We do not seek the continuation of tension with some European countries,” Iran’s president, Hassan Rouhani, said during a weekly cabinet meeting. “If Britain steps away from the wrong actions in Gibraltar, they will receive an appropriate response from Iran.”
Mr Rouhani spoke after a two-day visit by Iraq's Prime Minister, Adel Abdul-Mahdi, who said he went to Tehran this week partly at the request of Penny Mordaunt, the UK defence secretary at the time, to negotiate the release of Stena Impero.
Britain, the United States and other countries will meet at a US base in Florida on Thursday to discuss plans to protect ships in the Gulf from the threat of attack from Iran.
However, a regional official has expressed concern that competing maritime security plans from Europeans and Americans should not detract from efforts to de-escalate the current crisis . “We must ensure there is a window for a parallel political process to ensure freedom of navigation,” one Gulf official said.
Stena Bulk, the owner of the ship seized by Iran, said on Wednesday that it had spoken to all 23 crew members on board the ship and they were all well. Iran has released images of the crew on state television channels.
Nicholas Heras, a senior fellow at the Centre for New American Security, said Iran had identified the UK as a weak link among its foes in the region.
"Iran has identified the UK as a western country that it believes that it can push around," he told The National. "The Iranians would not dare to go this far into escalation with the United States, out of the fear that it would trigger a massive US military response.
“However, Iran looks at the UK as a weaker power that does not have the military resources to push back against Iranian bullying in the Gulf. The UK needs to demonstrate its military resolve to protect its maritime trade in the Gulf, or the Iranians will continue to escalate.”
Joel Gulhane, a Middle East analyst at The Risk Advisory Group, said the move probably would not deter Tehran from trying to harass other vessels in the region.
"While European states probably view this as securing freedom of maritime navigation, Iran is unlikely to welcome such deployments so further harassment of ships is likely," he said.
"For now it seems that the continued detention of Iranian oil on the Grace 1 tanker in Gibraltar places UK-flagged vessels at higher risk. But Iran may consider the seizure of Stena Impero as enough leverage to propose a swap."