The UK has agreed to a US request to use British military bases for "defensive" strikes on Iranian missile sites, Sir Keir Starmer has said.
But the prime minister said the UK had learned lessons from the "mistakes of Iraq", and was not involved in the initial strikes on Iran and "will not join offensive action now".
Sir Keir said the basis of the decision to accept the US request was the "collective self-defence" of allies and protecting British lives, accusing Iran of pursuing a "scorched-earth strategy".
The US will use the bases for the "specific and limited defensive purpose" of destroying Iran's missiles "at source", he added.
He said the move was in accordance with international law, and the government would publish a summary of its legal advice.
It was reported last month that the British government had not given permission for the US to use UK military bases to support any potential US strikes on Iran.
The US and Israel began attacking Iran early on Saturday, with US President Donald Trump encouraging the Iranian people to remove the country's government.
The Iranian regime has responded with attacks on US assets and countries in the region with a US military presence, including Bahrain, Qatar, the United Arab Emirates, Kuwait and Iraq.
On Saturday, Sir Keir said UK aircraft were "in the sky" in the Middle East as part of a defensive operation to protect its allies and citizens in the region, but said the UK "played no role" in the strikes.
In a video statement posted on social media on Sunday evening, Sir Keir said British aircraft had successfully intercepted Iranian strikes, but added: "Our partners in the Gulf have asked us to do more to defend them and it's my duty to protect British lives."
He began the statement by saying it "remains the case" that the UK is "not involved in the strikes on Iran".
At least 200,000 British citizens are in the region - including residents, those on holidays and passengers in transit, Sir Keir said - and the government "will continue to do all we can to support" them.
British people, including members of the armed forces, as well as allies, were being put at "huge risk" from Iranian strikes, he said, accusing the regime of "becoming even more reckless".
He added: "The only way to stop the threat is to destroy the missiles at source, in their storage depots, or the launchers which are used to fire the missiles.
"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, killing innocent civilians, putting British lives at risk, and hitting countries that have not been involved."
The UK, France and Germany, in a joint statement on Sunday evening, accused Iran of carrying out "indiscriminate and disproportionate" strikes.
The BBC understands UK government officials are also formulating plans to potentially evacuate British nationals from the Middle East, should airspace in the region remain closed.
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