The UK has agreed to let the US use British military bases to attack Iranian missile sites, Keir Starmer has said.

The UK has so far not been involved in the US-Israeli strikes on Iran, but in a recorded statement on Sunday evening, the prime minister said that Iran’s approach was becoming more reckless and putting British lives at risk, leading to the decision to allow the US to use two of its military bases.

The government also released a summary of its legal position, saying: “The UK is acting in the collective self-defence of regional allies who have requested support.”

Revealing that 200,000 British people are in Middle Eastern countries being targeted by Iran, Starmer said: “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 US 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.”

He said that British jets were in the air as part of coordinated defensive operations, which he said had “already successfully intercepted Iranian strikes”.

In a joint statement with France and Germany, the UK said on Sunday: “We will take steps to defend our interests and those of our allies in the region, potentially through enabling necessary and proportionate defensive action to destroy Iran’s capability to fire missiles and drones at their source.”

The UK’s deeper involvement in the conflict is likely to be controversial, with many in Labour and on the left, including Emily Thornberry, the Labour chair of the foreign affairs committee, saying the US’s action was in breach of international law.

Zack Polanski, the leader of the Green party, speaking to the BBC, said it was an “illegal and unprovoked attack”.

However, Starmer has also come under pressure from the Conservatives and Reform UK to offer more support to Trump.

The UK government has declined to say it supports Trump’s military action, which the US has said killed about 48 Iranian leaders, and refused to be drawn on whether this was lawful.

At the same time, it has declined to condemn the strikes, with the defence secretary, John Healey, saying on Sunday morning that few would mourn Iran’s supreme leader, Ali Khamenei.

“I think few people will mourn the ayatollah’s death, not least the families and friends of the thousands of protesters murdered on the streets of Iranian cities,” he said.

“Iran and the regime he’s led for so long, it’s a source of evil – murdering its own citizens and sponsoring and exporting terror.”