Boris Johnson is to chair a meeting of the National Security Council as Britain continued to urge all sides in the Gulf crisis to draw back from all-out conflict.
Foreign Secretary Dominic Raab spoke on Monday to his Iranian counterpart Mohammed Javad Zarif to stress the need for a diplomatic resolution to avoid a renewed conflict in the region.
Tehran has vowed “severe revenge” for the killing of its top military commander Qassem Soleimani in a US drone strike last week.
President Donald Trump in turn has warned the US was ready to strike back in a “disproportionate” manner if the Iranians hit US targets.
The Ministry of Defence refused to comment on reports it is stepping up contingency plans to evacuate military and civilian personnel from neighbouring Iraq amid fears they could be targets for Iranian reprisals.
The Times reported that a team of around 20 senior military planners and liaison officers had been dispatched to the embassy in Baghdad over the weekend.
Defence Secretary Ben Wallace is due to update MPs – returning to Westminster following their Christmas break – on the latest situation in the region.
The Prime Minister has faced criticism he was slow to respond to the crisis – only returning to the UK at the weekend following his new year break on the private Caribbean island of Mustique.
The former national security adviser Lord Ricketts said that ministers should have been quicker to react to the escalation.
“I think that the British Government were caught short by this,” he told BBC2’s Newsnight.
“People were still on holiday, and there wasn’t sufficient early recognition at the top level that this is a serious crisis and really dangerous for Western interests in the Middle East.
“Yes, I’m glad to see that the wheels are now in motion and things are happening but we are four days downwind of this sudden and dramatic escalation.”
Lord Ricketts said he would have expected the National Security Council to have met on Friday, when the killing of Gen Soleimani took place, to co-ordinate measures to ensure British nationals were protected.
“That may have been going on behind the scenes but there wasn’t, as far as I could see, a collective discussion among senior ministers.”
Mr Johnson has sought to tread a delicate diplomatic path – joining with French and German allies in calling for a “de-escalation” in the region.
At the same time, he is anxious to maintain good relations with the Trump administration ahead of talks with the US on a post-Brexit trade deal.
Mr Raab is due to fly to Washington later this week to meet Secretary of State Mike Pompeo after he criticised the lack of support by the Europeans – including the UK – for the US action.
The Foreign Secretary echoed Mr Johnson in saying the UK would not “lament” the passing of Gen Soleimani who he described as a “regional threat”.
However he also warned that a threat by Mr Trump to target Iranian cultural sites would be a breach of international law.
“We have been very clear that cultural sites are protected under international law and we would expect that to be respected,” he said.
Meanwhile US defence secretary Mark Esper was forced to deny the US was about to withdraw its troops from Iraq where the strike on Gen Soleimani took place.
A leaked letter from a US Marine Corps commander said that in “due deference to the sovereignty of the Republic of Iraq”, the coalition would be “re-positioning forces over the coming days and weeks to prepare for onward movement”.
It added: “We respect your sovereign decision to order our departure.”
It followed a vote in the Iraqi parliament on Sunday, backed by prime minister Adel Abdul-Mahdi, calling for the withdrawal of all foreign troops including British forces.
The Pentagon said the letter, addressed to the Iraqi defence ministry had been a “poorly worded” draft which should never have been released.
Mr Esper told reporters in Washington: “There’s been no decision whatsoever to leave Iraq. There’s no decision to leave, nor did we issue any plans to leave or prepare to leave.”
Mr Johnson emphasised the need to maintain a coalition presence in Iraq against Islamic State when he spoke by telephone to Mr Abdul-Mahdi on Monday.
Around 400 UK troops are stationed in Iraq in the fight against IS, while the US has 5,200, prompting fears of a withdrawal that could cripple the battle against the terror group.