Theresa May is to accuse Iran of "aggressive regional actions" in the Middle East, including stirring trouble in Iraq, Lebanon and Syria.
The Prime Minister will say she is "clear-eyed" about the threat posed by the country and will pledge to work with Gulf States to counteract its influence.
The Prime Minister will highlight last year’s nuclear deal as having "neutralised" the possibility of Iran acquiring weapons of mass destruction, but her tougher talk on the Middle Eastern country will also see her promise to confront anyone causing trouble.
In her speech at the Gulf Co-Operation Council, she was to say: "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 Conservative leader will accuse Iran of undermining stability in Lebanon and Iraq and will highlight how Iran has sent fighters to Syria to help prop up the Assad regime and provided support to the Houthis in Sanaa amid the conflict in Yemen.
The British Government, meanwhile, signed off £3.3bn of arms exports to Saudi Arabia while it was playing a key role in the first year of a brutal bombardment of Yemen. Oxfam warned today that Yemen will run out of food within months and its people are "at risk of catastrophic hunger" due to the conflict.
On the nuclear deal, the Prime Minister planned to say: "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 per cent 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."
Following intense six-way talks in Switzerland last year, Tehran agreed to reduce its uranium enrichment by 98 per cent in exchange for gradual relief of international sanctions as part of a deal that Donald Trump has said he may ditch.
The pact was signed by Iran and the P5+1 group of world powers; the US, UK, China, Russia, France and Germany.
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