Yvette Cooper has said that any talks between the US and Iran must result in a toll-free passage for vessels through the Strait of Hormuz.
The Foreign Secretary said that proposals have been circulating from Tehran to introduce tolls on the crucial waterway once the conflict concludes.
Ms Cooper was travelling to Japan on Sunday on the final leg of a diplomatic mission ahead of the US–Iran ceasefire expiring on Tuesday, which she called a “critical diplomatic moment”.
The Strait of Hormuz, through which a fifth of the world’s oil normally flows, has been closed by Iran as a response to America and Israel’s strikes.
In retaliation, US President Donald Trump began his own blockade of Iran’s oil ports, aimed at stemming Tehran’s fossil fuel income.
Mr Trump said that US negotiators will head to Pakistan on Monday for talks, lifting hopes of extending the ceasefire.
But on Sunday, the US said it had seized an Iranian-flagged cargo ship that it claimed was attempting to get around the blockade.
Mr Trump, in a post on social media, said the ship was warned by a US Navy guided missile destroyer in the Gulf of Oman to stop but it did not.
Iranian officials had earlier said that they remain open to negotiation but insisted ships will not pass through the Strait of Hormuz while the US blockade remains in place.
Mr Trump, announcing Monday’s talks, accused Iran of violating a ceasefire by targeting ships near the strait and threatened to destroy Iranian civilian infrastructure if Tehran rejects a US deal.
Ms Cooper said: “This is a critical diplomatic moment with the end of the ceasefire looming. Further talks on a lasting settlement are welcome – they must lead to a toll-free Strait of Hormuz.”
She added: “This argument is not just about the Strait of Hormuz, it is about the precedent this will set for freedom of navigation all over the world.
“If the wrong precedent is set, it would be deeply damaging not just for the global economy, but for global security, and that is why it is an argument we must win.”
Early on Monday, Iran’s first vice president Mohammad Reza Aref called for an end to economic and military pressures on Iranian oil exports, writing on X: “One cannot restrict Iran’s oil exports while expecting free security for others. The choice is clear: either a free oil market for all, or the risk of significant costs for everyone.”
In Tokyo, Ms Cooper is expected to focus on UK–Japan co-operation amid ongoing geopolitical uncertainty, including energy security and supply chain resilience.
She is set to meet Japan’s foreign minister Toshimitsu Motegi at the 10th UK–Japan Strategic Dialogue.
Ms Cooper has also met foreign ministers from 11 countries including Pakistan, Turkey, Egypt, the UAE and Ukraine, as well as speaking to US secretary of state Marco Rubio, during a “six-day diplomatic tour” ahead of the end of the ceasefire.
On Friday, Sir Keir Starmer said that the UK and France will lead a peaceful mission to protect freedom of navigation in the strait.
The Prime Minister said the mission would be put in place as soon as conditions allow, once hostilities in the Iran war are over.
Sir Keir and French President Emmanuel Macron hosted a meeting of world leaders in Paris on efforts to safeguard the flow of shipping.
But Mr Trump has 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.
