British Prime Minister Keir Starmer, who told parliament on Monday his government "does not believe in regime change from the skies", initially refused to have any role in the US's war with Iran.

He later agreed to a US request to use two British military bases for a "specific and limited defensive purpose".

The refusal sparked the ire of Trump who told British daily newspaper The Sun "this was the most solid relationship of all. And now we have very strong relationships with other countries in Europe", singling out France and Germany.

Starmer has cultivated a warm relationship with the unpredictable Trump, who was given an unprecedented second state visit to Britain last year.

The so-called special relationship between the World War II allies is largely built on long-standing defence co-operation and intelligence sharing.

Any potential military action in the Middle East, however, is politically sensitive in the UK following former prime minister Tony Blair's disastrous support for the US-led invasion of Iraq.

Trump said Starmer had "not been helpful", adding: "I never thought I'd see that. I never thought I'd see that from the UK. We love the UK."

"It's just a much different kind of relationship... It's very sad to see that the relationship is obviously not what it was," he said in the telephone interview.

- 'Tight diplomatic tightrope' -

The scathing comments come a day after Trump described Starmer's first reaction to the US request as "very disappointing" in an interview with another UK newspaper, The Daily Telegraph.

Government minister Darren Jones defended Starmer's decision to only get involved in military action where there was a "legal basis" to do so and a "clear plan" that was in the UK's own national interest.

"That's why we were not involved in the initial strikes in Iran in the Middle East," he said.

"But it is also why we've consented to American Air Forces using our air bases and for British jets to be in the sky in order to defend British citizens who are currently in the region," he added.

He said two UK bases -- one in Gloucestershire in western England and the UK-US Indian Ocean Diego Garcia base -- had now been cleared for use by the Americans.

Starmer's Downing Street office said on Monday he took the decision after Iran fired missiles over the weekend that put British interests and people "at risk".

"We all remember the mistakes of Iraq, and we have learnt those lessons," Starmer said later in parliament.

The prime minister added that the British Royal Air Force (RAF) base at Akrotiri in Cyprus -- where the runway was hit early Monday by an unmanned drone -- was "not being used by US bombers".

British politicians have been haunted by the 2003 Iraq war in which a reported 179 UK soldiers died.

An official UK inquiry into the conflict later found that Blair acted on flawed intelligence when deciding to join the war.

Evie Aspinall, director of the British Foreign Policy Group think tank said Starmer faced a "very tight diplomatic tightrope".

"The UK wants to support the United States, not least to protect its own security interests and to show European value to the US, which remains critical in the context of Ukraine and Greenland," she told AFP.

"At the same time, it is deeply cautious of engagement with a conflict that could rapidly increase global insecurity and of working closely with an increasingly unreliable United States, who keeps undertaking actions which don't align with UK interests," she added.