The UK and France will lead a multinational mission to protect commercial shipping routes in the Strait of Hormuz, Sir Keir Starmer has said.
Speaking after a meeting of 51 countries, the prime minister underlined the work would be "strictly peaceful and defensive" and would only be put in place once fighting in the region ends.
Iran effectively blocked the waterway since the US and Israel attacked the country in late February, driving up global energy and fuel prices.
On Friday, Iran's foreign minister said the route was now "completely open" and US President Donald Trump made a similar announcement and criticised NATO for being "useless when needed".
Trump said he received a call from NATO offering help on Friday but, in a post on Truth Social, he wrote that he "TOLD THEM TO STAY AWAY, UNLESS THEY JUST WANT TO LOAD UP THEIR SHIPS WITH OIL".
The US president has repeatedly criticised NATO, and more specifically the UK, during the Iran war.
Sir Keir's position has been that the UK would not be "dragged" into the conflict and he previously said the UK would not support the blockade.
Meanwhile, the UK prime minister said dozens of countries had offered to "contribute assets" to the joint mission with France.
"This will be strictly peaceful and defensive, as a mission to reassure commercial shipping and support mine clearance," he said alongside French President Emmanuel Macron.
"We invite all nations with an interest in the free flow of global trade to join us. Some have already indicated their readiness to contribute."
Sir Keir confirmed more details of the mission will be announced next week, following a military planning conference in London.
Tehran promised the Strait of Hormuz will be completely open to commercial vessels until the middle of next week, when a temporary ceasefire between the US and Iran will end.
Trump said his own naval blockade of vessels moving in and out of Iranian ports would remain in force.
While there was a flurry of shipping activity in the Gulf throughout Friday, maritime tracking services showed very few vessels transiting through the strait.
Macron said that the closure of the strait has had "very severe consequences" on "the whole of the planet and the global economy".
"Recent events are encouraging, even if we have to remain prudent," he said.
German Chancellor Friedrich Merz said that Germany "stands ready to play its part in ensuring freedom of navigation".
Speaking after the multilateral meeting, he also said that it would be "desirable" for the US to be involved.
BBC