Royal Navy warships have been ordered to escort British-flagged vessels in the Persian Gulf, in the wake of the US killing of Iran’s top military leader.
Defence Secretary Ben Wallace said it would help protect ships and citizens amid fears Iran will seek revenge for the assassination of Qasem Soleimani.
Boris Johnson is due to return to the UK after a 12-day holiday and will talk to foreign leaders in the coming days.
General Soleimani was killed this week in a US drone strike in Baghdad, Iraq.
The killing of General Soleimani marks a major escalation in tensions between the US and Iran, with Tehran vowing to avenge his killing.
US President Donald Trump last night tweeted that the US will strike 52 Iranian sites “very fast and very hard” if Tehran follows through with its vow of revenge.
Ben Wallace said HMS Montrose and HMS Defender will accompany UK-flagged ships through the Strait of Hormuz, as they did between July and November following the seizure of the Stena Impero by Iran.
He said he had spoken to his US counterpart, Mark Esper, on Friday and urged all parties to de-escalate the situation.
But Mr Wallace added: “Under international law the United States is entitled to defend itself against those posing an imminent threat to its citizens.”
Mr Johnson is expected to return to the UK later after spending almost two weeks on the Caribbean island of Mustique.
He has yet to speak publicly about the US airstrike or subsequent threats from Iran.
Outgoing Labour leader Jeremy Corbyn renewed his criticism of the PM for not cutting short his holiday and called for an urgent meeting of the Privy Council – the group that advises the Queen – over the airstrike.
Shadow foreign secretary Emily Thornberry had called the UK government’s response “pathetic”, while other party leaders have urged Mr Johnson to clarify the UK’s position.
Meanwhile, Downing Street said the Foreign Secretary Dominic Raab will travel to Washington this week to meet US counterpart Mike Pompeo.
Mr Pompeo had criticised America’s European allies for not being “helpful” in the wake of the assassination . However, he later said in a tweet that he was “thankful that our allies recognise the continuing aggressive threats posed by the Iranian Quds Force”.
Mr Raab is expected to meet his French and German counterparts before travelling to the US capital on Thursday.
Following the strike, the Foreign Office has hardened its travel advice for Britons in Iraq and Iran. Officials also urged those travelling to the United Arab Emirates, Turkey and Israel to “remain vigilant”.
In its advice, published on Saturday, the Foreign Office said there is a risk that British or British-Iranian dual nationals “could be arbitrarily detained or arrested in Iran”.
“The criminal justice process followed in such cases falls below international standards,” it said.
The Foreign Office also said alerts for other parts of the Middle East were being increased, with calls for citizens to “remain vigilant” in nations including Afghanistan, Israel, Lebanon, Saudi Arabia and the United Arab Emirates.
It advised people to keep up to date with developments via the media and its own travel advice.
It comes as the US has pledged to send 3,000 extra troops as a precaution. The UK has 400 troops based in the Middle East and works alongside US forces in the region.
On Saturday, around 150 people gathered outside Downing Street for an “emergency” protest organised by the Stop the War Coalition, urging the US to avoid more conflict with Iran.