Britain has evacuated its Iranian embassy amid fears of an imminent US strike.
The British ambassador to Tehran and all other consular staff vacated the embassy on Wednesday following an assessment of the security situation.
The closure was described as temporary by the Foreign Office.
British troops, along with their American counterparts, were also withdrawn from the al-Udeid US-run air base in Qatar ahead of potential military action.
An RAF plane arrived at the Qatari base on Wednesday morning and was flying back to the British base at Akrotiri in Cyprus hours later, according to flight tracking data.
Other US troops have also been moved out of bases in Iraq, out of fear of a potential retaliatory Iranian attack.
Officials warned on Wednesday that the US could strike within 24 hours, with two European officials telling Reuters that military intervention was probable.
Regional allies were told that a strike was “more likely than not,” a US official told the Wall Street Journal.
An Israeli official also said it appeared Mr Trump had made a decision to take action, though the scope and timing were not yet clear.
On Wednesday night, Iran closed its airspace to nearly all flights.
The Islamic Republic issued a notice to air mission (Notam) shortly after 1.30am local time (10pm GMT) blocking all flights except international arrivals and departures with prior government permission.
Iran’s airspace rapidly emptied shortly after the announcement of the notice, which was due to last for at least two hours, with only five aircraft visible over the country, FlightRadar24 data show.
The Foreign Office also warned against all but essential travel to Israel amid the prospect of escalating violence in the region.
Despite this, Mr Trump played down the threat of imminent military action, telling reporters in the Oval Office that he had been assured the killing of protesters had stopped.
Asked if he was still considering an intervention, Mr Trump added: “We’re going to watch and see what the process is but we were given a very good, very good statement by people that are aware of what’s going on.”
On Wednesday, there was a flurry of diplomatic activity, with Marco Rubio, the US secretary of state, calling Benjamin Netanyahu, the Israeli prime minister, and Saudi Arabia warning Washington that any attempt to topple the Islamic Republic in Tehran would destabilise the region and disrupt global oil markets.
