The prime minister said the UK would not join offensive strikes by Israel and the US on Iran, but defended his decision late on Sunday to permit the US to conduct defensive strikes on Iranian missile sites from RAF bases, saying that was “the best way to protect British interests and British lives”.

As MPs urged Starmer not to allow the UK to be dragged further into the conflict, Starmer suggested he had qualms about the US action and plans in place for the aftermath of the strikes.

“We all remember the mistakes of Iraq, and we have learned those lessons. Any UK actions must always have a lawful basis, and a viable thought-through plan,” he said. “That is the principle that I applied to the decisions that I made over the weekend.”

He said he would not risk the lives of British military personnel unless an operation had a “proper lawful basis”.

The US president said on Monday he was “very disappointed” in Starmer for blocking him from using two British bases to carry out strikes on Iran, adding: “It sounds like he was worried about the legality.”

Speaking in the Commons, Starmer said the UK was deploying planes and allowing the use of bases for defensive purposes because of Iran launching strikes on the UK’s allies in the region in its retaliation.

“It is clear that Iran’s outrageous response has become a threat to our people, our interests and our allies, and it cannot be ignored,” Starmer said. “The basis for our decision is the collective self-defence of longstanding friends and allies and protecting British lives. It is in accordance with international law.”

He said the RAF had intercepted an Iranian drone strike heading for a coalition base in Iraq where UK forces were stationed. Two drones were also fired at the British base in Cyprus, RAF Akrotiri. Starmer said they were launched before Sunday’s night’s statement on the US use of UK bases.

The defence secretary, John Healey, said the damage was minimal but that the UK was moving families who lived at RAF Akrotiri to alternative temporary accommodation in Cyprus. The base is not being used to launch US strikes.

A Cypriot news agency reported the damage to the airbase was caused by small low-flying drones launched by Hezbollah, an Iranian proxy group from Lebanon – but the UK Ministry of Defence did not formally confirm that.

Starmer had not previously expressed explicit opposition to Saturday’s initial wave of US-Israeli attacks on Iran, which killed Ayatollah Ali Khamenei, the country’s supreme leader, and other senior Iranian leaders.

Iran has since launched a wave of retaliatory missile and drone attacks on a range of targets in the Gulf and the Middle East, including the UAE, Saudi Arabia, Kuwait, Qatar, Iraq, Bahrain and Oman.

The UK is expected to allow the US to use RAF Fairford in Gloucestershire and Diego Garcia in the Chagos Islands to bomb Iran’s “missile cities”, sites where high-speed ballistic missiles, Iran’s most dangerous weapons, are stored and can be launched from.

The hits would require the use of US heavy bombers using so-called bunker-buster munitions, with the British bases preferred by the US air force because they are closer to Iran than the US itself.

The Conservative leader, Kemi Badenoch, said Starmer was using the cover of international law to justify his weak position – and accused him of making a U-turn after the scale of Iran’s retaliation became clear.

“Today, the president of the United States has taken the extraordinary step of rebuking the prime minister publicly, saying he took far too long to grant access,” she said. “We are told that this dither and delay is because of concerns over international law. But I’m afraid that explanation simply does not hold. International law didn’t prevent our allies from clearly and unequivocally stating whose side they were on.”

Starmer said: “We believe that the best way forward for the region and for the world is a negotiated settlement in which Iran agrees to give up any aspirations to develop a nuclear weapon and ceases its destabilising activity across the region. That has been the longstanding position of successive British governments.

“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; that is what I’ve done, and I stand by it.”

The war in the Middle East in maps, video and photos

But, he said, the UK could not stand by while UK citizens in the Gulf as well as British bases were put at risk. He said there were an estimated 300,000 British citizens in the region, including those in transit, and airports and hotels where British citizens were staying were being hit.

He said Gulf leaders had been asking the UK to do more to defend them. “Moreover, it is my duty, the highest duty of my office, to protect British lives,” he added.

Starmer said that it was not possible for British Typhoons and F-35s to intercept all the Iranian strikes and that was why he had given the US permission to destroy the missiles in their storage depots or at launch sites.

He added that it would “prevent Iran firing missiles across the region, killing innocent civilians, putting British lives at risk and hitting countries that have not been involved”.

The prime minister said he wanted to make sure British citizens stranded because of closed airspace could “ensure that they can return home as swiftly and safely as possible”. He said the Foreign Office would send rapid deployment teams to the region, especially the UAE, and that British citizens should register their presence.