Sir Keir Starmer will say reopening the Strait of Hormuz is a “global responsibility” as he co-hosts a gathering of world leaders in Paris on Friday.
The crucial shipping lane, through which a fifth of the world’s oil normally flows, has been closed by Iran as a response to America and Israel’s strikes.
In retaliation, US President Donald Trump has begun his own blockade of Iran’s oil ports, aimed at stemming Tehran’s fossil fuel income.
Sir Keir will arrive in Paris late on Friday morning to co-host a virtual meeting with Emmanuel Macron and then have lunch with the French president.
Around 40 countries and the International Maritime Organisation are expected to be on the call.
The Prime Minister is expected to tell the summit: “The unconditional and immediate reopening of the strait is a global responsibility, and we need to act to get global energy and trade flowing freely again.
“Emmanuel Macron and I are clear in our commitment to establish a multinational initiative to protect freedom of navigation.
“We must reassure commercial shipping and support mine clearance operations to ensure a return to global stability and security.”
Foreign Secretary Yvette Cooper and Chief of the Defence Staff Sir Richard Knighton will join the Prime Minister.
The call will focus on supporting the US-Iranian ceasefire and coming up with a plan to ensure the Strait of Hormuz can remain open to shipping after the Middle East war ends.
Planning for a joint defensive mission is under way with a view to countries sending a combined military effort as soon as conditions allow, Downing Street has said.
The conference on Friday will be followed up by a multinational military planning summit at Permanent Joint Headquarters in Northwood, north-west London, next week.
Foreign Secretary Yvette Cooper is on her own diplomatic offensive in the run-up to the US-Iran ceasefire expiring on Tuesday and is due to visit Turkey and the Gulf as well as Japan.
