The Royal Navy will protect British ships travelling through the Strait of Hormuz amid heightened tensions in the region, after a US air strike killed a top Iranian general.
Type 23 frigate HMS Montrose and Type 45 destroyer HMS Defender are the two Navy warships tasked with escorting UK-flagged ships through the strait.
"The Government will take all necessary steps to protect our ships and citizens at this time," Defence Secretary Ben Wallace said in a statement on Saturday.
Iran warned the US of "severe revenge" for the attack on General Qassem Soleimani, who was in charge of Iran's military operations abroad.
The US said he was killed for orchestrating attacks on American soldiers and bases.
Mr Wallace urged "all parties" to de-escalate the situation but also said the US was "entitled to defend itself".
“General Soleimani has been at the heart of the use of proxies to undermine neighbouring sovereign nations and target Iran’s enemies," Mr Wallace said.
"Under international law, the United States is entitled to defend itself against those posing an imminent threat to their citizens."
Until November, the Royal Navy was escorting ships through the Strait of Hormuz after the British-flagged Stena Impero tanker was seized by Iran in July.
The Ministry of Defence says UK ships now have a choice to navigate the waters without an escort at their own risk.
The two warships are already deployed to the Gulf, alongside four Royal Navy minehunters.
It comes as the Foreign Office strengthened its travel warnings across the region as fears of war increase.
Shadow Chancellor John McDonnell condemned the Government's call for de-escalation, saying "it's not good enough".
"What we expect the UK Government to do is to come out in total and outright condemnation of this act of violence," he told a crowd from the Stop the War Coalition group.
US President Donald Trump has warned Iran of any retaliation, saying the US could hit 52 Iranian sites "very fast and very hard".
Mr Trump also said the strike against Gen Soleimani was "to stop a war" rather than "start a war".
Since the attack, the US has deployed 3,000 extra troops to Kuwait.
Outgoing Labour leader Jeremy Corbyn wrote to the Prime Minister, calling for an urgent meeting of the Privy Council - the group that advises the monarch.
He said he wanted to know if the "assassination" had heightened the terror risk to the UK.
Boris Johnson, who has been celebrating the New Year in the Caribbean, is yet to comment on the attack.
Cover Image: HMS Montrose escorts two British ships through the Strait of Hormuz in July, last year (Picture: Royal Navy).