Sir Keir Starmer has spoken with the Iranian president as part of international efforts to de-escalate tensions in the Middle East, Sky News understands.
The 30-minute phone conversation with Masoud Pezeshkian followed a joint statement on Monday issued with the US, France, and Germany calling on Iran and its allies to "stand down its ongoing threats of a military attack against Israel".
There are growing fears Iran will retaliate against Israel over the killing of a senior Hamas member, prompting the US to order the deployment of a guided missile submarine to the Middle East.
Read More: Middle East on brink of dangerous escalation
Sky's Middle East correspondent Alistair Bunkall said it had been a "busy night of diplomacy" for Sir Keir, who warned Iran during the call that any action on their part risks pushing the region into an "uncontrollable war".
He said it was a "very rare occurrence" for a British prime minister to speak to an Iranian leader and it "shows the value" of having a British embassy in Tehran - not something many countries, including the US, have.
In the call, Sir Keir was able to push the message Western leaders are using "as the carrot.. to bring Iran down from the edge", which is to say "look, if we can try and push forward with a ceasefire deal and ceasefire talks are due to take place on Thursday, then anything that you do in the in between that or even after that runs the risk of collapsing those talks", Alistair said.
In the earlier joint statement, which also included Italy, the world leaders said there "is no further time to lose" in reaching a ceasefire and securing the release of the remaining hostages in Gaza.
"All parties must live up to their responsibilities," they said.
"In addition, unfettered delivery and distribution of aid is needed.
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"We expressed our support for the defense of Israel against Iranian aggression and against attacks by Iran-backed terrorist groups.
"We called on Iran to stand down its ongoing threats of a military attack against Israel and discussed the serious consequences for regional security should such an attack take place."
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