Prime Minister David Cameron will hold face-to-face talks with Iranian President Hassan Rouhani as part of a bid to revive the Syrian peace process.
The pair will meet on Tuesday in New York, where they are both attending a United Nations summit.
Mr Cameron said he hoped to bolster regional support for a political settlement in Syria.
Efforts to engage Tehran in a push to end Syria’s civil war have been spurred by a thaw in relations with the west.
Last month, the British embassy in Iran reopened nearly four years after it was closed after it was stormed by protesters during a demonstration against sanctions.
And in July, Tehran reached a deal with six world powers aimed at curbing its nuclear programme.
Iran has been a close ally of Syrian President Bashar al-Assad in recent years.
Mr Cameron is expected to drop his opposition to President Assad playing a role in any transitional government during talks with a number of leaders in New York.
But on Sunday he told reporters President Assad had “butchered his own people” and was “one of the great recruiting sergeants” for so-called Islamic State.
“He can’t play a part in the future of Syria and that position hasn’t changed,” the PM continued.
“Obviously conversations about how we bring about transition are very important and that’s what we need to see greater emphasis on.”
Asked if he believed Mr Assad should face prosecution at the International Criminal Court, Mr Cameron said: “People who break international law should be subject to international law.”
Mr Cameron is attending several sessions at the UN but will not join Mr Rouhani, US president Barack Obama, Russian president Vladimir Putin and China’s Xi Jinping in making a keynote speech.
Labour leader Jeremy Corbyn criticised that decision.
“What matters now is a broad and comprehensive plan as the foundation for a political solution to the conflict and a new United Nations Security Council resolution,” he said.
“That’s why it is so disappointing that David Cameron isn’t showing leadership and unlike other world leaders won’t be speaking at the UN this week.”