Sir Keir Starmer has said he is ready to lift sanctions on Iran once a peace deal between the US and Tehran is finally signed.
Donald Trump declared a memorandum of understanding had been reached last night - with a deal set to be finalised on Friday.
And just before midnight, the Prime Minister said Britain was "ready to support the technical talks" which will soon begin.
He pledged to "work with partners", then "work with international partners", and help lead a mission to clear mines in the Strait of Hormuz - though only "in an agreed way".
Shortly afterwards, he and the leaders of France, Germany and Italy also reacted to the breakthrough.
The so-called E4 group said they "are prepared to lift relevant sanctions in response to clear, verifiable steps by Iran on its nuclear programme".
"We will work intensively with the US, Iran and regional partners to seize this moment," a statement declared - with a list of carve-outs for international law.
"This is a moment of opportunity to restore regional stability and stabilise the global economy," it said.
"It is now vital that the detailed negotiations are concluded and this agreement is implemented rapidly and comprehensively. We are ready to support that effort.
"The urgent re-opening of the Strait of Hormuz with unconditional and unrestricted freedom of navigation is essential.
"We are committed to playing our part to achieve this - in accordance with our respective constitutional requirements - including through a strictly defensive and independent mission to reassure commercial shipping and conduct mine clearance operations."
Donald Trump did not mention Britain or Europe in his remarks after the breakthrough.
Though he did acknowledge that the strait would open up "for purposes of mine removal" on Friday.
More to follow...
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