British foreign minister Jeremy Hunt said he had spoken to his Iranian counterpart Mohammad Javad Zarif and expressed his "extreme disappointment" over Iran's seizure of a British-flagged tanker in the Strait of Hormuz on Friday.
"Just spoke to ... Zarif and expressed extreme disappointment that having assured me last Saturday Iran wanted to de-escalate situation, they have behaved in the opposite way," he tweeted on Saturday.
"This has to be about actions not words if we are to find a way through," Hunt added. "British shipping must and will be protected."
The Foreign Office in London said it summoned Iran's charge d'affaires on Saturday.
The diplomat was called in as European powers urged Iran to release the Stena Impero oil tanker it seized in the Strait of Hormuz on Friday.
The government's COBRA emergencies committee met late Friday and was expected to meet again on Saturday.
London-based maritime security risk analysts Dryad Global said the "dramatic escalation" by Iran was likely to have been a calculated move.
The interception was "likely to have been a pre-planned operation and intended for use at a time where Iran perceives there to be strategic narrative to be leveraged", it said.
The maritime experts said Iran was likely targeting what it perceived as the "junior" partner to the United States.
"The UK will be forced to respond, but will find few avenues to do so, both militarily and diplomatically," the firm said.
