Theresa May is throwing her political weight behind the Iran nuclear deal after sustained attacks on the agreement by US President-elect Donald Trump.
The Prime Minister is using a speech at the Gulf Co-operation Council gathering in Bahrain on Wednesday to assure regional leaders she is "clear-eyed" about the threat posed to them by Iran, and to insist the nuclear arrangement with Tehran must stand.
On the final day of her trip to Bahrain, Mrs May will tell Gulf leaders: "We secured a deal which has neutralised the possibility of Iran acquiring nuclear weapons for over a decade. It has already seen Iran remove 13,000 centrifuges together with associated infrastructure and eliminate its stock of 20% enriched uranium.
"That was vitally important for regional security. But we must also work together to push back against Iran's aggressive regional actions, whether in Lebanon, Iraq, Yemen, Syria or in the Gulf itself."
Mrs May's stance on the Iran nuclear deal puts her at odds with Mr Trump, who has strongly attacked the agreement.
His hostility to the deal saw outgoing CIA director John Brennan warn him that abandoning it would be "disastrous" and the "height of folly".
During his campaign for the White House, Mr Trump pledged to "dismantle" the deal which sees international sanctions lifted in return for Tehran limiting its nuclear programme.
The Prime Minister will also tell the Gulf summit: "As we address new threats to our security, so we must also continue to confront state actors whose influence fuels instability in the region.
"So I want to assure you that I am clear-eyed about the threat that Iran poses to the Gulf and the wider Middle East, and the UK is fully committed to our strategic partnership with the Gulf and working with you to counter that threat."
The PM, who is leading a discussion on Iran at the summit, will also tell the leaders she is under no illusion about Tehran's aggressive interventions in Syria and Yemen.