The president, a longtime critic of Nato, has stepped up criticism after allies refused to join the US-Israel war on Iran

Donald Trump has said he is “absolutely” considering withdrawing the US from Nato, warning that the matter was “beyond reconsideration” after the refusal of US allies to join the US-Israeli war against Iran.

The president’s threats, his most determined to date, have left the alliance facing its worst crisis in its 77-year history, a former US ambassador has warned.

Trump has long been vocally sceptical about the benefit of Nato membership to the US, but since North Atlantic allies have refused to take part in the month-long, faltering US-Israeli assault on Iran, the president has stepped up his rhetoric.

He told Reuters news agency on Wednesday he was “absolutely without question” considering withdrawal, after telling the Telegraph the matter was “beyond reconsideration”, insisting he had never been “swayed by Nato”. He signalled that he would express his disgust for Nato in an address to the nation scheduled for Wednesday evening.