LONDON -- Britain is not joining the US and Israeli "offensive strikes" on Iran, Prime Minister Keir Starmer said on Monday.

Speaking to the British parliament, Starmer said making decisions like this requires "a lawful basis for what the United Kingdom is doing" and "a viable thought-through plan."

He said this is the principle he applied to the decision not to get involved in the "offensive strikes" of the United States and Israel on Iran, adding, "This government does not believe in regime change from the skies."

"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 said.

US President Donald Trump said he is "very disappointed" in Starmer for blocking him from using the military base in Diego Garcia to carry out strikes on Iran, according to British media reports on Monday.

Britain's previous refusal to let US forces use the base was unlike anything that had "happened between our countries before," Trump said.

In a statement on Sunday night, Starmer accepted the US request to use the base for "specific and limited defensive purposes." But Trump said Starmer "took far too long" to change his mind.