Prime Minister Sir Keir Starmer “stands by” his decision not to join in with the strikes on Iran carried out by the United States and Israel.
The UK leader was speaking in the House of Commons, as he updated MPs and the public on the latest situation in the Middle East.
Sir Keir restated that the UK was not involved in the initial strikes carried out on Saturday.
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The prime minister said he had faced criticism from US President Donald Trump for his decision not to join in the strikes.
“It is my duty to judge what is in Britain’s national interest,” Sir Keir said.
“That is what I’ve done, and I stand by it.”
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The prime minister also appeared to challenge whether Mr Trump had a plan for the next stage of the conflict in the Middle East when the offensive began.
Several times, Sir Keir referred to the need for a “lawful basis” and a “viable, thought-through plan” before committing British service people to action.
“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 is a viable thought-through plan with an objective that can be achieved or has a viable prospect of being achieved.
“That is the principles that I apply to the decisions that I made over the weekend, the principles I applied to the decision not to get involved in the offensive strikes of the US and Israel.”
Sir Keir said he took two decisions over the weekend – one not to join in with the initial strikes, and then one to allow US planes to conduct defensive actions from UK bases.
Hours after Sir Keir announced on Sunday he had allowed US use of UK bases, a drone hit RAF Akrotiri in Cyprus.
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Regime change from the skies?
Speaking to The Daily Telegraph earlier, Mr Trump criticised the first decision.
“That’s probably never happened between our countries before,” the president said.
“It sounds like he was worried about the legality.”
Speaking about the latter decision to allow US use of British bases, Mr Trump said: “It is useful. It took far too much time. Far too much time.”
Kemi Badenoch, the leader of the Conservatives, criticised the prime minister for his “dither and delay” in joining in the action against Iran.
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Another Tory MP, Sir Edward Lee, asked what evidence there was Iran was about to acquire nuclear weapons, and whether “regime change from the skies” ever worked.
Sir Keir said: “This government does not believe in regime change from the skies.”
