Sir Keir Starmer has refused to join Donald Trump’s naval blockade of the Strait of Hormuz.
The US president announced that he would stop tankers from entering or leaving the key oil and gas shipping lane and said other countries would help.
However, The Telegraph understands that Britain will not play a role in enforcing the blockade.
After US-Iranian peace talks in Pakistan ended without a deal, Mr Trump lashed out at Iran for its refusal to open the waterway.
He wrote on his Truth Social platform that the US navy would start “blockading any and all ships trying to enter, or leave, the Strait of Hormuz”.
Mr Trump added, without elaborating: “Other countries will be involved with this blockade.”
Downing Street said the UK was “urgently working with France and other partners to put together a wide coalition to protect freedom of navigation”.
Britain has mine-hunting systems in the region that could be used to help clear the strait of naval mines laid by Iran, Sir Keir said.
Meanwhile, Mr Trump continued his attacks on the Prime Minister by again comparing him to Neville Chamberlain, whose premiership was defined by his 1930s appeasement of Adolf Hitler.
“Nato is shameful,” the US president told Fox News. “I mean, look at the United Kingdom … PM Starmer said ‘we’ll send the equipment after the war is over’.
“I said ‘you don’t need equipment when the war is over. You need the equipment before the war starts, or during the war’…
“He made a public statement that ‘we will send equipment after the war is over’, that’s a Neville Chamberlain statement.”