The UK and France will lead a peaceful mission to protect freedom of navigation in the Strait of Hormuz, Sir Keir Starmer said.
The Prime Minister said the mission would be put in place as soon as conditions allow, once hostilities in the Iran war are over.
But US President Donald Trump belittled the efforts of Nato nations, saying the transatlantic allies had been “useless when needed” and were only getting involved now the situation was over.
Sir Keir and French president Emmanuel Macron hosted a meeting of world leaders in Paris on efforts to safeguard the flow of shipping through the vital oil and gas route when the fighting ends.
While the talks took place in the French capital, Iran has said it has reopened the strait to shipping, in a move welcomed by Mr Trump.
Sir Keir said: “I can confirm that along with France, the UK will lead a multinational mission to protect freedom of navigation as soon as conditions allow.
“This will be strictly peaceful and defensive, as a mission to reassure commercial shipping and support mine clearance.
“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.
“We will take this forward with a military planning conference in London next week where we will announce more detail on the composition of the mission, and over a dozen countries have already offered to contribute assets.”
But Mr Trump posted on his Truth Social platform: “Now that the Hormuz Strait situation is over, I received a call from NATO asking if we would need some help. I TOLD THEM TO STAY AWAY, UNLESS THEY JUST WANT TO LOAD UP THEIR SHIPS WITH OIL.
“They were useless when needed, a Paper Tiger!”
The conflict in the Middle East has strained transatlantic relations, with Mr Trump repeatedly condemning Nato allies for refusing to support his military operations against Iran.
Iran had effectively closed the Strait of Hormuz since the US-Israeli bombing campaign began at the end of February, forcing up energy and fuel prices around the world.
Tehran said it had reopened the strait to shipping as part of the ceasefire deal in the Middle East.
The US and Iran are in the middle of a fragile truce which is due to last until April 22, while Israel and Lebanon have begun a 10-day ceasefire.
Iran’s foreign minister Abbas Araghchi said: “In line with the ceasefire in Lebanon, the passage for all commercial vessels through Strait of Hormuz is declared completely open for the remaining period of ceasefire, on the co-ordinated route as already announced by Ports and Maritime Organisation of the Islamic Rep of Iran.”
Mr Trump said Tehran had announced the “strait of Iran” is “fully open and ready for full passage”.
Iran, with the help of the USA “has removed, or is removing all sea mines”, he said.
But the US blockade of Iranian ports would remain “in full force and effect” until “such time as our transaction with Iran is 100% complete”.
The price of oil fell sharply and stock markets globally rallied higher after the announcement.
Oil benchmark Brent crude fell 11% to just over 88 US dollars a barrel on the back of the news.
Sir Keir said: “We welcome the announcement that was made during our meeting, but we need to make sure that that is both lasting and a workable proposal and, if anything, it reinforces the need for the work that we’ve been doing this afternoon, because we’re very clear that the mission that we’re putting together is a defensive mission, and that comes after a ceasefire.”
Mr Macron said: “Recent events are encouraging, even if we have to remain prudent.”
