Sir Keir Starmer will declare reopening the Strait of Hormuz a “global responsibility” as he co-hosts a gathering of world leaders in Paris.
The crucial shipping lane, vital for a fifth of global oil supplies, was closed by Iran following America and Israel’s strikes.
In retaliation, US President Donald Trump has begun blockading Iran’s oil ports, aiming to stem Tehran’s fossil fuel income.
Sir Keir will arrive in Paris late Friday morning for a virtual meeting with Emmanuel Macron, followed by lunch with the French president.
Around 40 countries and the International Maritime Organisation are expected 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.