EPASir Keir Starmer has told MPs that the government "does not believe in regime change from the skies", putting him at odds with President Trump over the joint US-Israeli strikes on Iran.
In his first statement to Parliament since the strikes, Sir Keir defended his decision not to permit the use of UK bases for the initial wave of attacks.
"President Trump has expressed his disagreement with our decision not to get involved in the initial strikes, but it is my duty to judge what is in Britain's national interest," he told MPs.
However, the situation changed on Sunday when Iran's "outrageous" response became "a threat to our people, our interests and our allies", the PM said.
Iran's retaliation to the US and Israeli attacks had threatened British people across the Middle East, prompting the decision to allow the bases to be used to hit Tehran's missile infrastructure, he said.
He also cited a drone attack on RAF Akrotiri in Cyprus, which he said was "not being used by US bombers".
On Saturday, the US and Israel struck Iran, killing the country's Supreme Leader Ayatollah Ali Khamenei.
Iran has responded by launching missiles and drones across the region, targeting countries including Israel, Bahrain, Kuwait, Qatar, the United Arab Emirates, Saudi Arabia and ships passing through the Strait of Hormuz.
The UK refused permission for the US military to use British bases for what the prime minister called "offensive strikes"; but subsequently agreed for the use of British military bases for "defensive" strikes on Iranian missile sites.
He told MPs: "To be clear, the use of British bases is limited to the agreed defensive purposes; we are not joining the US and Israeli offensive strikes."
He added: "The lessons of history have taught us that it is important when we make decisions like this, that we establish there is a lawful basis for what the United Kingdom is doing.
"That is one of the lessons from Iraq, and that there's a viable thought-through plan with an objective that can be achieved or has a viable prospect of being achieved.
"That is the principle that I applied to the decisions that I made over the weekend."
He added: "This government does not believe in regime change from the skies."
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