Keir Starmer has said Israel and Iran "need to get back" to the ceasefire deal as the agreement appeared to have collapsed.
It comes as tensions ramp up after Donald Trump accused both sides of breaking an agreement brokered overnight. In a sweary rant, the US President claimed the warring nations "don't know what the f*** they're doing".
He had earlier declared the 12 day war, which saw Israel and the United States target Iranian nuclear facilities, was "over".
But in a dramatic statement shortly before noon UK time, Trump accused both nations of violating the ceasefire. Israel has vowed to respond "forcefully" as the risk of escalation rises.
Mr Starmer said: “Well, I want the ceasefire to continue, and therefore, obviously, the sooner we get back to that, the better. And that's the message that I'm discussing with others leaders today. We need to get back to that ceasefire, which is consistent with what I've been saying about deescalation for quite some time now."
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And referring to an Iranian bombing of a US base in Qatar, the PM said: “I should just say that I exchanged messages with the Emir of Qatar yesterday as well in relation to the attack yesterday.”
Trump, speaking to reporters as he left the White House for the NATO summit in the Netherlands, said: "They (Iran) violated it but Israel violated it too." Trump said. He added: "I'm not happy with Israel."
He said: "I'm not happy with them. I'm not happy with Iran, either, but I'm really unhappy with Israel going out this morning. We basically have two countries that have been fighting so long and so hard that they don't know what the f*** they're doing."
This morning Israeli Prime Minister Benjamin Netanyahu said his government had agreed to a bilateral ceasefire with Iran. Netanyahu claimed to have achieved all his war goals in an update to his cabinet.
But within hours the agreement appeared to have been broken, with Israeli defence minister Israel Katz saying Iran had "completely violated" the ceasefire.
Mr Katz said he instructed the military to resume targeting Iranian paramilitary and government targets after missiles were launched by Tehran. Iran's military denied firing missiles at Israel after the agreement came into force.
Earlier Trump announced an "Official END" to the war. He posted on the Truth Social platform: "THE CEASEFIRE IS NOW IN EFFECT. PLEASE DO NOT VIOLATE IT!"
In Israel, at least 28 people have been killed and more than 1,000 wounded in the war. Israeli strikes on Iran have killed at least 974 people and wounded 3,458 others, according to the Washington-based group Human Rights Activists.
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