The prime minister has said Iran must "face up to the consequences" of its actions after a drone attack on an Israeli-linked oil tanker in which two crew members, including a British national, were killed.
Speaking after Iran's ambassador was summoned to the Foreign Office, Boris Johnson said: "This was clearly an unacceptable and outrageous attack on commercial shipping."
The ambassador was summoned this morning for a meeting with Foreign Office minister James Cleverly after the UK and US blamed Iran for the strike.
"Minister Cleverly reiterated that Iran must immediately cease actions that risk international peace and security, and reinforced that vessels must be allowed to navigate freely in accordance with international law," a spokesperson said.
The UK and US have blamed Iran for the attack on the Israeli-linked oil tanker, in which two crew members - a British national and a Romanian citizen - were killed.
The attack happened last Thursday when the tanker MV Mercer Street was off Oman's coast in the Arabian Sea.
It is the first known fatal attack after years of assaults on commercial shipping in the region linked to tensions between Israel and Iran.
Foreign Secretary Dominic Raab accused Iran of carrying out a "deliberate, targeted" assault, which constituted "a clear violation of international law", and has promised to work with allies on a "concerted response".
US Secretary of State Anthony Blinken also said he was considering "next steps" with the UK and other allies, with "an appropriate response... forthcoming".
The meeting with Mr Cleverly follows a weekend of discussions among high-level officials on how best to respond to the incident.
The head of the British armed forces, General Sir Nick Carter, held discussions with his Israeli counterpart on Sunday.
Meanwhile, the government held a Cobra emergency meeting at the level of officials over the weekend in a sign of the serious focus on the tanker attack, Sky News understands.
There's not yet been a Cobra attended by ministers, which is what happens in the gravest of crises.
The Mercer Street tanker is managed by London-based Zodiac Maritime, part of Israeli billionaire Eyal Ofer's Zodiac Group.
Earlier this week, Israeli foreign minister Yair Lapid blamed "Iranian terrorism" for the attack.
Tehran has denied responsibility, accusing Israel of making baseless accusations against Iran.
The Foreign Office said the drone assault followed similar attacks on three other Israeli-linked ships in the region since February and there are concerns that tensions are increasing in the region.
A UK source said crew members reported being targeted by "some sort of drone" on Thursday in the Arabian Sea, before communications with the ship were lost.
If a drone attack is confirmed it would raise speculation about a possible link to a government or some kind of proxy group.
Iran in the past has repeatedly been accused of targeting tankers in the Gulf. The country has been locked in a tit-for-tat contest with Israel, with both sides accusing the other of targeting ships and launching cyber attacks.