The US president had previously suggested a deal was close to being finalised.
Donald Trump said he will not rush into a deal with Iran, a day after saying an agreement had been “largely negotiated”.
The US president said “both sides must take their time” as “there can be no mistakes”.
Sir Keir Starmer said any deal should mean the Strait of Hormuz being reopened with “unconditional and unrestricted” access for shipping.
— Rapid Response 47 (@RapidResponse47) May 24, 2026
The strait has been effectively closed to most shipping by Iran since the US-Israeli bombing campaign began at the end of February.
The closure of the strait, which normally carries around 20% of global oil and gas shipping, has driven up prices around the world.
Writing on his Truth Social platform, Mr Trump said: “The negotiations are proceeding in an orderly and constructive manner, and I have informed my representatives not to rush into a deal in that time is on our side.”
He said the US blockade of Iran’s coast will continue “until an agreement is reached, certified, and signed”.
Mr Trump said: “Both sides must take their time and get it right. There can be no mistakes!
“Our relationship with Iran is becoming a much more professional and productive one. They must understand, however, that they cannot develop or procure a Nuclear Weapon or Bomb.”
On Saturday Mr Trump had indicated that a deal was close, saying “final aspects and details of the deal are currently being discussed, and will be announced shortly”.
The Prime Minister said the UK would work with allies to “seize this moment” to help achieve a long-term solution in the Middle East.
Sir Keir said: “I welcome the progress towards an agreement between the US and Iran.
“We need to see an agreement that brings the conflict to an end and reopens the Strait of Hormuz, with unconditional and unrestricted freedom of navigation.
“It’s vital that Iran must never be allowed to develop a nuclear weapon.
“My government will continue to do everything we can to protect British people from the impact of this conflict.
“We will work with our international partners to seize this moment and achieve a long-term diplomatic settlement.”
The UK has already announced Royal Navy destroyer HMS Dragon has been sent to the Middle East to prepare for a possible international mission to safeguard shipping in the strait if a peace deal is put in place.

The UK and France have been leading efforts to pull together an international alliance willing to provide reassurance and assist with mine clearance once the hostilities cease.
Armed forces minister Al Carns visited RFA Lyme Bay in Gibraltar, a support vessel which can act as a mothership for mine-hunting drones which could be sent to the Gulf.
But Chief Secretary to the Prime Minister Darren Jones played down the prospect of an imminent peacekeeping mission.
He told Sunday Morning With Trevor Phillips on Sky News: “I’m not sure that that’s on the table right now.
“We have long-standing defence partnerships with countries in the region, anyway, as well as British military bases there, which we’ve talked a lot about in the last few weeks, as this conflict has flared up.
“But I think what is really the question here is how we get the Strait of Hormuz open, how the Americans and others can help to clear mines that may have been laid in the Strait of Hormuz, so that commercial shipping companies have the confidence again to be able to travel through it.”
A deal between Tehran and Washington could end the war and reopen the strait.
The US wants Iran give up its stockpile of highly enriched uranium, but details of how that is achieved may be pushed to further negotiations.
In exchange, the US could allow Iran to sell its oil through sanctions waivers.

