The Foreign Secretary says the UK believes it is highly likely Iran is behind a "deliberate, targeted and unlawful" attack on an oil tanker off the coast of Oman.
Two people, including a British security guard, were killed in the incident.
Dominic Raab said in a statement: "UK assessments have concluded that it is highly likely that Iran attacked the MV MERCER STREET in international waters off Oman on 29 July using one or more Unmanned Aerial Vehicles (UAVs).
"The UK is working with our international partners on a concerted response to this unacceptable attack."
Israel alleges Tehran carried out the assault on the oil tanker amid heightened regional tension.
On Sunday, Iranian Foreign Ministry spokesman Saeed Khatibzadeh described the allegation Iran carried out the attack as "baseless".
"Such blame games are nothing new," Mr Khatibzadeh said. "Those who are responsible for this (attack) are the ones who made it possible for the Israeli regime to set its foot in this region."
He added: "This is not strange, the well-known anti-Iran lobby in the US uses any opportunity to make… accusations against the Islamic Republic of Iran."
Thursday's strike on the ship, M/T Mercer Street, is thought to be the first fatal attack after years of assaults on commercial shipping in the region linked to tension with Iran over its tattered nuclear deal.
The vessel is managed by London-based Zodiac Maritime – part of Israeli billionaire Eyal Ofer's Zodiac Group.
The firm said two crew members – one British and one Romanian – were killed but believes no other crew members were harmed.
UK maritime security group Ambrey said one of its team members was killed on board the ship.
The M/T Mercer Street, empty of cargo, had been on its way from Dar es Salaam in Tanzania to Fujairah in the United Arab Emirates, when it was hit, Zodiac Maritime said.
It was north-east of the Omani island of Masirah, 185 miles south-east of Oman's capital Muscat.
The stricken ship was escorted by US aircraft carrier USS Ronald Reagan and destroyer USS Mitscher to safe port, according to the US Navy's 5th fleet based in the Middle East.
Cover image: PA.