The Foreign Office has hardened its travel advice for Iran and Iraq after the killing of top Iranian general Qasem Soleimani in a US airstrike.
It warns British nationals not to travel to Iraq, except for essential travel to the Kurdistan Region of Iraq, and only essential travel to Iran.
The death has increased tensions in the region, where the UK has 400 troops.
The Royal Navy has resumed escorting British ships in the Gulf, where a tanker was seized last year.
Defence Secretary 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 spoke to his US counterpart Mark Esper on Friday and urged all parties to de-escalate the situation.
But Mr Wallace said: "Under international law the United States is entitled to defend itself against those posing an imminent threat to its citizens."
US forces in Iraq were "repeatedly attacked" by Iranian-backed militia and Gen Soleimani was "at the heart" of such efforts to undermine neighbouring nations and target Iran's enemies, he added.
Earlier Shadow foreign secretary Emily Thornberry had called the UK government's response "pathetic".
Ms Thornberry said: "I don't understand why Britain isn't calling for an emergency meeting of the [UN] security council.
"I think there is a tremendous risk we could end up in another war and I think we have to do everything that we can to pull Donald Trump back from this."
Labour leader Jeremy Corbyn has written to Prime Minister Boris Johnson calling for an urgent meeting of the Privy Council - the group that advises monarchs - over the airstrike.
Meanwhile, former foreign secretary Jeremy Hunt said the situation was "very, very risky" and the UK's job was to "use our influence to argue for consistent US policy".
The killing of Gen Soleimani marks a major escalation in tensions between Washington and Tehran.
There were chants of "death to America" in the streets of Baghdad on Saturday as mourners took part in a funeral procession for him.
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.
Foreign Secretary Dominic Raab said: "Given heightened tensions in the region, the FCO now advise people not to travel to Iraq, with the exception of the Kurdistan Region of Iraq, and to consider carefully whether it's essential to travel to Iran. We will keep this under review."
Its advice for British-Iranian dual nationals remains unchanged, warning them not to travel to Iran.
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.
Mr Raab has previously urged for a calming of tensions, adding: "Further conflict is in none of our interests."
'We need to stand up to US'
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.
Among them were shadow chancellor John McDonnell and shadow justice secretary Richard Burgon.
Speaking on BBC Breakfast, Ms Thornberry - who has said she will run for the Labour leadership - called the airstrike a "deliberate and reckless escalation of the conflict with Iran".
"I'm very concerned we could get dragged into this," she said.
"We have a place on the security council, not so that we can play at being Donald Trump's mini-me or always agreeing with the Americans. There are times we need to stand up to them."
BBC News understands the prime minister was not warned about Friday's airstrike.
Speaking on BBC Radio 4's Today programme, former foreign secretary Mr Hunt said he thought it was "regrettable because, as one of the US's closest allies, I think it's an important aspect of that relationship that there are no surprises".
"But it may also have been because they didn't want to put us in a difficult position of asking us to make a judgement as to whether we agreed or not with what was done."
He added: "The UK cannot afford to be neutral if we want to be a serious global player."
Writing to the prime minister, Mr Corbyn asked several questions including what the UK was told before the airstrike and what the government was doing to ensure the safety of UK nationals.
Earlier, he called the killing of Gen Soleimani a "US assassination" and said the UK "should urge restraint" from both Iran and the US.
Mr Corbyn became a member of the Privy Council in 2015. Its members include all former prime ministers and cabinet ministers as well as leaders of the opposition.
Scottish First Minister Nicola Sturgeon described the situation as "extremely worrying" and urged all parties to "restrain from further violence and work to de-escalate tensions".
"Further conflict in the Middle East is in no-one's interests," she said.
The acting leader of the Liberal Democrats, Sir Ed Davey, called for an immediate statement from Mr Johnson about the UK's position, adding that Britain should "work with a broader group of concerned states at the United Nations".
Mr Raab spoke to US Secretary of State Mike Pompeo on Friday, although the time of the call is not known.
Mr Pompeo tweeted that he was "thankful that our allies recognise the continuing aggressive threats posed by the Iranian Quds Force".
In a statement, Mr Raab urged "all parties to de-escalate" after the killing of Gen Soleimani.
He said the UK "recognised the aggressive threat" Gen Soleimani posed, but "further conflict is in none of our interests".