"The United States has requested permission to use British bases for that specific and limited defensive purpose. We have taken the decision to accept this request to prevent Iran firing missiles across the region," he said in a video message on X.
Starmer reiterated that Britain was not involved in the joint US-Israel air strikes on Iran that killed the country's Supreme Leader Ali Khamenei on Saturday, adding that Britain would not join further waves of strikes.
But he said Iran had retaliated by launching sustained attacks across the region and its missiles had hit airports and hotels where British citizens were staying.
"Our decision that the UK would not be involved with the strikes on Iran was deliberate, not least because we believe that the best way forward for the region and for the world is a negotiated settlement, one in which Iran agrees to give up any aspirations to develop a nuclear weapon," he said on Sunday.
"But Iran is striking British interests nonetheless, and putting British people at huge risk."
Britain's partners in the Gulf had asked it to do more to defend them, he said, and it was his duty to protect British lives.
British fighter jets were already taking part in coordinated defensive operations and had intercepted Iranian strikes, he said, but the only way to stop the threat was to destroy the missiles at their source in storage depots or launchers.
Accepting the US request, therefore, was based on the "collective self-defence of long-standing friends and allies and protecting British lives" in accordance with international law, he said.
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