Sir Keir Starmer has demanded that Donald Trump end his blockade of the Strait of Hormuz, which is set to cause economic pain for Britons as petrol costs soar.
The prime minister had already announced that “we are not supporting the blockade” and the UK is “not getting dragged in” to the Iran war on Monday morning.
But Sir Keir then announced that he was bringing 40 nations together to force the reopening of the Strait, which Iran had initially blockaded and demanded a reversal in the White House’s policy.
He made clear in a statement to parliament that he condemned the actions of Iran in blockading the straits but refuses to support Trump’s response of blockading it himself.
In a direct challenge to President Trump and Iran, he said: “The ongoing closure of the Strait of Hormuz is deeply damaging. Getting global shipping moving is vital to ease cost of living pressures.
“The UK has convened more than 40 nations who share our aim to restore freedom of navigation.
“This week, the UK and France will co-host a summit to advance work on a coordinated, independent, multinational plan to safeguard shipping when the conflict ends.”
The prime minister will discuss cost of living pressures with local people on a visit to Greater Manchester on Monday, while chancellor Rachel Reeves will travel to Washington for International Monetary Fund meetings this week, after warning that “the war in Iran will come at a cost to British families and business”.
The US president threatened to stop tankers from entering or leaving the key oil and gas shipping lane, a move that is expected to further drive up oil prices when markets open, after they have already risen as a result of Iran’s grip on the strait in retaliation for the US-Israel war against it.
The latest developments come as MPs return to Westminster from the Easter recess on Monday with no resolution to the Middle East crisis in sight and the fate of a shaky two-week ceasefire uncertain.
Mr Trump announced the shipping blockade after US-Iranian peace talks in Pakistan ended without a deal, with both sides blaming each other.
The American leader said on his Truth Social platform that the US military would start “blockading any and all ships trying to enter, or leave, the Strait of Hormuz” from 10am US Eastern Time.
Mr Trump added, without elaborating: “Other Countries will be involved with this Blockade.” However, it is understood that Britain will not take part.
Sir Keir took a firm line on the blockade on Monday morning, telling BBC 5Live: “All the time the strait is shut or not free for navigation in the way it should be, that means that oil and gas is not getting to market.
“That means the price is going up, and that means everybody listening to this is facing higher energy bills, and I don’t want that to happen.”
The prime minister added: “We’re not supporting blockade”, promising that all of Britain’s diplomatic and military efforts will go towards “getting the strait fully open”.
It came as a government minister repeatedly refused to say whether or not the UK agrees with Trump’s blockade of the strait.
Asked whether the government supports the blockade, education minister Olivia Bailey told Sky News: “We won’t be taking any part in it, and we want to see free navigation of the Strait of Hormuz. That’s a really important principle. We’re working with our international allies to do that.”
Pressed on the issue, she repeatedly said: “We are not going to be taking part in it, and we want to see free navigation of the Strait of Hormuz.”
And asked whether the government is worried the blockade could send oil prices soaring, Ms Bailey said the impact on personal finances is a “huge concern”.
She said: “We’re working very hard to see the Strait of Hormuz open, because we want to ensure that we are really mitigating the damage on people’s pockets which we’re seeing.
“That’s a huge concern for us, and so we’re working very hard to do that.”
Sir Keir spoke with French president Emmanuel Macron over the weekend, with the two leaders agreeing on the need to work with a broad coalition of partners to “protect freedom of navigation”, a government spokesperson said.
Britain will host further talks on reopening the maritime pinch-point with a coalition of countries this week.
The third such meeting convened by the UK is expected to look for ways to support a sustainable end to the conflict and focus on increasing international diplomatic pressure on Iran to reopen the strait, including through sanctions.
Sir Keir’s call with Mr Macron on Sunday took place before Mr Trump posted on social media about a blockade, it is understood.
Mr Trump told Fox News “the UK and a couple of other countries are sending mine sweepers” to the strait, and “it won’t take long to clean it out”.
The prime minister previously said UK mine hunting systems were already in the region. But this is thought to refer to minesweeping drones, which could be deployed once the situation stabilises, and to be distinct from Mr Trump’s blockade.
US Central Command, which is responsible for American military operations in the Middle East, said its blockade of Iranian ports would begin on Monday.
US forces “will not impede freedom of navigation for vessels transiting the Strait of Hormuz to and from non-Iranian ports,” the military said in a statement that appeared to contradict Mr Trump’s threat to stop all ships.
The president blamed the failure of negotiations on Iran’s refusal to reopen the channel and commit to giving up its nuclear ambitions.
Iran’s foreign minister Abbas Araghchi accused Washington of “maximalism, shifting goalposts, and blockade,” adding: “Enmity begets enmity.”
Mr Trump has also renewed his war of words with Pope Leo XIV, who previously criticised the president’s rhetoric regarding the conflict.
In a post on Truth Social early on Monday, Mr Trump said: “Pope Leo is WEAK on Crime, and terrible for Foreign Policy… I don’t want a Pope who thinks it’s OK for Iran to have a Nuclear Weapon.”
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