"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," British Foreign Secretary Dominic Raab said.
"We believe this attack was deliberate, targeted, and a clear violation of international law by Iran."
He said British assessments had concluded it was "highly likely" that Iran attacked the tanker using one or more drones.
"The UK is working with our international partners on a concerted response to this unacceptable attack," he said.
(Reporting by Paul Sandle, Editing by William Maclean)