Foreign Secretary Yvette Cooper is set to host talks with a coalition of countries on Thursday to reopen the crucial Strait of Hormuz shipping lane.
Donald Trump, who was due to give a televised address on Wednesday night, has meanwhile made contradictory statements about whether he was ready to wind down the Iran war or escalate it.
Britain is seeking to lead a diplomatic initiative, understood to include 35 countries such as France, Germany and some Gulf nations, to reopen the passage, but Sir Keir Starmer has admitted restoring the flow of global trade will not be easy.
The virtual meeting will look at “all viable diplomatic and political measures” to reopen the vital oil and gas shipping route, which Iran has effectively closed for most exports.
Sir Keir outlined plans for the meeting to Nato secretary general Mark Rutte and German Chancellor Friedrich Merz in phone calls on Wednesday afternoon.
Following the UK-led meeting, military planners will consider how to make the strait “accessible and safe” after the fighting has stopped, although this is not expected to involve the deployment of Royal Navy warships to police the waterway.
Mr Trump had previously indicated he will be prepared to end military operations without the strait being reopened to shipping traffic, but said on Wednesday he would only consider a ceasefire if the strait is reopened.
The US President also said Iran had requested a ceasefire, which Tehran said was “false and baseless”.
Sir Keir attempted to reassure anxious Britons that the Government has a plan to deal with the cost-of-living pressures caused by the war with a press conference on Wednesday.
He also said Britain would push forward with its pursuit of closer ties with the EU in the wake of the oil crisis, as the transatlantic relationship comes under strain from Mr Trump’s repeated criticism of allies for refusing to join the military action.
The US president told the Daily Telegraph he is considering whether to pull America out of Nato in response to the perceived lack of support.
Sir Keir said in response that he would continue to act in the British national interest “whatever the pressure on me and others, whatever the noise”.
Chancellor Rachel Reeves, meanwhile, has said she is “angry” that Donald Trump launched the Iran war without a “clear plan” and that it would have implications for the UK’s economy.
