Dominic Raab has accused Iran of murdering a British citizen in a "deliberate" attack on an oil tanker.
The Foreign Secretary vowed to draw up a “concerted response” with allies after the deadly assault on the high seas which saw two people killed.
The currently unnamed British security guard and a Romanian national - understood to be the ship's captain - on board the MV Mercer Street died in the drone strike off the coast of Oman on Thursday.
Mr Raab said the UK has concluded that "it is highly likely" Iran carried out an "unlawful and callous attack".
The Japanese-owned Liberian-flagged tanker managed by Israeli-owned Zodiac Maritime, was targeted by what US officials described as "suicide drones".
The unmanned aircraft loaded with explosives self-detonated upon reaching their target 175 nautical miles from land in the Arabian Sea.
The explosion ripped through the bridge and living quarters.
The British security guard was employed by Ambrey Ltd, a spokesperson for the firm has told the Mirror.
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In a statement, Mr Raab said today: "The UK condemns the unlawful and callous attack committed on a merchant vessel off the coast of Oman, which killed a British and a Romanian national.
"Our thoughts are with the friends and family of those killed in the incident.
"We believe this attack was deliberate, targeted, and a clear violation of international law by Iran.
"UK assessments have concluded that it is highly likely that Iran attacked the MV Mercer Street in international waters off Oman on July 29 using one or more unmanned aerial vehicles (UAVs)."
The minister went on to warn Iran it "must end such attacks" and vessels must be allowed to navigate freely in accordance with international law.
He added that the UK is working with international partners on a "concerted response to this unacceptable attack".
"Iran must end such attacks, and vessels must be allowed to navigate freely in accordance with international law. The UK is working with our international partners on a concerted response to this unacceptable attack."
The 28,400-tonne Mercer Street was sailing from Dar es Salaam, Tanzania to Fujairah in the United Arab Emirates with no cargo onboard when she was targeted.
The US Navy Fifth Fleet said explosive experts believed a drone strike targeted the 600ft tanker.
An American warship escorted her to safety.
London-based Zodiac Maritime is owned by Israeli billionaire Eyal Ofer.
The ship was attacked in a move seen as a further escalation in the proxy war unfolding between Tel Aviv and Tehran.
Israeli Prime Minister Naftali Bennett said over the weekend that he knew "with certainty" that Tehran was to blame.
Hinting at retaliation, he added: “We know how to send a message to Iran.”
But the Iranians issued a furious denial.
Foreign Ministry spokesman Saeed Khatibzadeh said: "The Zionist regime (Israel) has created insecurity, terror and violence.
“These accusations about Iran's involvement are condemned by Tehran.
"Such accusations are meant by Israel to divert attention from facts and are baseless.”
At least three other Israeli-linked ships have been attacked in the same region over the past five months.
They include the Bahamas-flagged vehicle carrier Helios Ray on February 25 off the coast of Oman, the Hyperion Ray vehicle carrier on April 13 off the coast of UAE and the container ship Tyndall off the UAE on July 2.
An Ambrey spokesperson confirmed the British security guard worked for the firm but said no name was available.
"It is with deep regret that I can confirm that an Ambrey team member has tragically been killed in a security incident," a statement said.
"The incident occurred in the northern Indian Ocean onboard the product tanker M/T Mercer Street.
"We can confirm a member of the crew was also killed.
"We are currently working closely with our client and the relevant authorities, whilst offering all the support possible to the victim’s next of kin.
"Our thoughts are with the family and friends of those involved at this incredibly sad time."
