Top security firms have removed British guards from ships in the Gulf because of fears that Iran could try to capture UK nationals as tensions soar in the Middle East. Ambrey, the biggest company in the sector, and Maritime Asset Security and Training (MAST) said they had replaced UK citizens with guards from other countries. The move is a response to Iran’s dramatic seizure of a UK-flagged tanker, the Stena Impero, three weeks ago in the Strait of Hormuz, the world’s most important shipping lane for oil and other fuels.

The vessel and its 23 crew members, none of them British, continue to be held by Iran. Its foreign minister this week said Tehran would launch legal action over the vessel’s alleged breaches of maritime law. John Thompson, a former elite UK soldier who co-founded UK-based Ambrey, said: “We have instigated a policy of no UK guards in the Gulf. [We] are advising our clients the same.” Ben Stewart at MAST said: “We’ve been recommending not using UK unarmed guards because of that [risk].”

The maritime security industry, which flourished when the threat of piracy off the coast of Somalia rocketed ten years ago, has traditionally been dominated by former members of elite British forces, such as the Royal Marines, Parachute Regiment, SAS and SBS. In recent years the industry has turned to cheaper guards from eastern Europe and Asia as the sector has become more competitive, but British guards are still often used as senior members of security teams. Rising tensions in the Gulf stem from US president Donald Trump’s decision last year to withdraw from a landmark 2015 nuclear deal with Iran, which has left Washington and Tehran at loggerheads.

A series of security incidents began in May when four tankers, including two Saudi vessels, were hit by “sabotage” attacks in the Gulf of Oman. US officials said they suspected Iran, but Tehran denied any involvement. The incidents have triggered a rush to put guards on ships in the region to help with lookouts and security procedures. Unlike in zones of Somali piracy, guards have gone unarmed because of the risk of coming into conflict with Iranian military forces.

Fears for vessels in the Gulf, where approximately 20 percent of the world’s oil supplies pass every day, have already led the US and the UK to establish escorts for their tankers and other ships. The UK has two warships currently deployed in the Gulf. People in the shipping industry and a US government official have said there are concerns that Iran could be trying to target UK nationals.

Stena Bulk, the Sweden-based owner of the Stena Impero, has said its crew members are Indian, Russian, Filipino and Latvian. They remain on board the ship, which has been parked in waters off the coast of Iran. Some Iranian officials have said the Stena Impero was seized in retaliation for the UK’s detention of an Iranian supertanker off Gibraltar last month. The UK said the Grace 1 supertanker was planning to breach EU sanctions against Syria. “Everyone in the UK shipping industry seems to believe the Iranians want British crew,” one person said. They asked not to be named given the tensions surrounding shipping. “That’s the impression we have, given the events that have happened and the feedback from people on board.” Patrick Rogers at S-RM, a global risk consultancy that advises a number of major shipping companies, said the decision to remove UK guards was sensible.

“If it’s a higher risk to have them on board then why would you?” Mr. Rogers said. “You’re at risk of provoking them.” The US maritime agency said on Thursday that US-flagged ships should inform US and British naval authorities if they planned to sail into the Gulf. Britain this week aligned itself with the US by joining an American-led task force for escorting ships through the Gulf and nearby waters. Big European countries other than the UK have been wary of joining US-led freedom of navigation effort, in part for fear of appearing to be supporting the US’s hardline approach to Iran.

The EU has publicly opposed Mr. Trump’s decision last year to pull out of the Iran nuclear deal. One senior EU diplomat said that — while the British viewed Washington’s involvement in the protection operation as inevitable and necessary because of its military capabilities — other countries wanted to guard the EU’s “strategic autonomy”. David Balston at the UK Chamber of Shipping said the escorting of UK-flagged ships was “providing considerable reassurance” but stressed that it was an issue for all countries operating in the waters of the Gulf. “We do welcome the positive engagement of the UK government and the broader internationalisation of the issue. This is not a UK only problem. It is very much an international problem,” Mr Balston said.