Sir Keir Starmer has warned Britons risk paying the price for America's military blockade of the Strait of Hormuz just hours before Donald Trump shuts down the key shipping route.
The Prime Minister will likely drive an even bigger wedge between himself and the US President, with Mr Trump already lashing out at him over his response to events in the Middle East.
His declaration comes as the US President announced the blockage of the key shipping route is set to begin in just hours.
"We are not supporting the blockade," Sir Keir told the BBC.
"All of the marshalling diplomatically, politically and capability - we do have minesweeping capability - that's all focused on getting the straits open.
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"That's the way we get energy prices down as quickly as possible."
The Prime Minister emphasised that ordinary Britons are bearing the financial burden of the ongoing crisis in the vital shipping lane.
"What we've been doing is bringing countries together to keep the straits open and not shut," he explained, noting that Gulf States had expressed their desire for unimpeded maritime passage during his recent regional visit.
Rather than deploying Royal Navy vessels and troops to enforce the American blockade on Iranian ports, the UK is channelling all its resources towards restoring full access to the waterway.
The US military blockade of the Strait of Hormuz is set to begin today
| GETTYBritish minesweepers and anti-drone systems will maintain their presence in the region, though Sir Keir declined to elaborate on specific operational details.
The Prime Minister has consistently ruled out any direct British military engagement in the wider conflict.
"We're not supporting the blockade and all of the marshalling diplomatically, politically and capability - that's all focused, from our point of view, on getting the Strait fully open," he said.
Having just returned from meetings with Gulf leaders, Sir Keir stressed that his administration's entire strategy centres on collaboration with allies to ensure the shipping channel remains accessible rather than closed.
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Washington announced the blockade after diplomatic negotiations between the US and Iran collapsed without producing a lasting peace agreement for the Middle East.
Following talks between American and Iranian representatives in Islamabad, Mr Trump declared that US forces would begin "blockading any and all ships trying to enter, or leave, the Strait of Hormuz".
US Central Command subsequently also confirmed it would enforce the measures "impartially" against vessels travelling to and from Iranian ports in both the Arabian Gulf and Gulf of Oman.
However, it will permit the free passage of ships heading to other destinations.
Sir Keir Starmer has declared that Britain will not participate in America's military blockade of the Strait of Hormuz
| GETTYThe strategic waterway handles approximately one-fifth of the world's oil and liquefied natural gas trade, effectively blocked since US-Israeli military strikes commenced on February 28.
Brent crude surpassed $100 per barrel after the blockade was announced.
The UK Government is pressing ahead with efforts to assemble a broad international coalition alongside France to safeguard maritime freedom in the strait.
"We are urgently working with France and other partners to put together a wide coalition to protect freedom of navigation," a Government spokesman confirmed.
Officials have also rejected reports that Iran is attempting to impose $2million transit charges on tankers navigating the waterway, insisting that vessels must not face any tolling requirements.
Sir Keir spoke with President Emmanuel Macron to coordinate their joint approach to the crisis.
Meanwhile, Chancellor Rachel Reeves is expected to travel to Washington this week for the IMF meeting.
Ms Reeves is expected to push her counterparts for the reopening of the Hormuz shipping route.