Sir Keir Starmer has called on Iran to “return to the negotiating table” after the US bombed nuclear sites in the country overnight.
In a statement, the UK prime minister said stability in the region was a priority, describing Iran’s nuclear programme as “a grave threat to international security”.
The BBC understands there was no UK involvement in the US strikes on Iranian nuclear facilities and that the prime minister was informed of them in advance.
“Iran can never be allowed to develop a nuclear weapon and the US has taken action to alleviate that threat,” Starmer added.
The US said it had carried out strikes on three nuclear facilities in Iran: Natanz, Isfahan and Fordo on Saturday night.
It followed the launch of a new Israeli operation against Iran on 12 June. Israel says its targets were military sites, including nuclear facilities.
Iran launched retaliatory strikes – with the two countries continuing to exchange fire since.
President Donald Trump had initially refused to say whether the US would get involved, with the White House saying on Friday that a decision would be made “within the next two weeks”.
In a televised address following the strikes, the president said the operation was a “spectacular military success”, adding that if Iran did not make peace quickly it would face “far greater” attacks.
Iran’s foreign minister Seyed Abbas Araghchi called the US strikes “outrageous” and said they would have “everlasting consequences”.
“Each and every member of the UN must be alarmed over this extremely dangerous, lawless and criminal behavior,” he added.
UN Secretary General António Guterres said the overnight strikes were a “dangerous escalation”.