Sir Keir Starmer and G7 counterparts have welcomed Donald Trump's "breakthrough" deal with Iran and voiced support for a "robust and comprehensive" agreement to follow it.
In a joint declaration, the leaders of the US, UK, France, Germany, Italy, Japan and Canada said: "We recognise the breakthrough and the opportunity that currently exist in the Middle East."
The deal provides a "historic opportunity to prevent Iran from acquiring any nuclear weapon and tackling the threats related to its regional and ballistic activities," they said, adding that they are ready to contribute to implementing it.
The group also backed the UK-French led effort to facilitate shipping through the key shipping lane, the Strait of Hormuz, and called for verification that all mines are removed.
However, they stressed that a follow-up agreement needed to be negotiated, flagging that further talks were required on Iran's nuclear programme.
"We strongly support a robust and comprehensive diplomatic follow-on agreement to the Memorandum of Understanding secured by President Trump that can bring peace and security for all in the region," the declaration stated.
"We underline the need for the negotiation to this end to address the threats posed by Iran in the region and beyond and ensure that they never obtain a nuclear weapon."
"We agree that such a negotiation would benefit from the contributions from relevant regional and international partners, including the International Atomic Energy Agency (IAEA)."
"We reaffirm that Iran will never obtain a nuclear weapon."
Sir Keir Starmer has said the UK will play its full part in reopening the Strait of Hormuz but has not specified when British support could arrive.
The G7 leaders also reiterated "unwavering" support for Ukraine in their statement.
Sir Keir said he had spoken to Donald Trump about potentially reimposing sanctions on Russian oil but was unsure if the US president had made a decision.
The US in March temporarily eased some sanctions on Russian oil shipments as crude prices sharply increased. The waiver has been extended.
Speaking to ITV on Wednesday, the Prime Minister said: "We had the discussion, I don't know that a decision has been made yet, but we had a very open, actually a very constructive discussion about Ukraine."
He noted "a real sense of a mood change" on Ukraine, attributing it to recent advances by Kyiv and damage to the Russian economy from a series of sanctions by the UK and allies.
