Britain is sending warships to patrol the Strait of Hormuz to "protect its citizens".
It comes just days after the US assassinated Iranian Quds Force chief General Qassem Suleimani and Iran-backed militia leader Abu Mahdi Al Muhandis in a rocket attack on Baghdad Airport.
In a statement, Defence minister Ben Wallace said Britain’s navy will accompany UK-flagged ships to provide protection after the killings by US forces.
Mr Wallace said he had ordered the warships HMS Montrose and HMS Defender to prepare to return to escort duties for all ships sailing under a British merchant flag.
“The government will take all necessary steps to protect our ships and citizens at this time,” he said.
Mr Wallace said he had spoken to his US counterpart Mark Esper on Saturday.
He added: "We urge all parties to engage to de-escalate the situation.
"Under international law the United States is entitled to defend itself against those posing an imminent threat to their citizens."
The UK had announced last August that it was joining the US-led naval mission to protect vessels travelling through the Strait of Hormuz.
It move came after Iran seized a British-flagged vessel, the Stena Impero, in the Strait of Hormuz last July for alleged marine violations.
The seizure came two weeks after British forces detained an Iranian oil tanker, the Grace 1, in Gibraltar over suspicions it was taking oil to Syria in breach of EU sanctions.
The naval coalition was formed in response to increased tension in the Strait of Hormuz after attacks on oil tankers, which the US blamed on Iran.