The King used Tuesday night's historic address to the US Congress to hail Nato's vital role and the need to defend Ukraine.

In a 20-minute speech to lawmakers, Charles became the first British King, and only the second British monarch after his mother, to address a joint session. 

He received a four-minute standing ovation as he walked into the chamber, and another round of applause as he rose to speak.

Standing at a lectern in the historic chamber of the Hall of the House of Representatives, the King delivered a speech packed with charm and humour as well as some hard truths.

Referring to his grandfather, King George VI, who became the first reigning British sovereign to set foot in America in 1939, he also referenced Anglo-US co-operation during the Second World War.

'Today, we find ourselves in a new era, but those values remain,' he said. 'It is an era that is, in many ways, more volatile and more dangerous than the world to which my late mother spoke, in this chamber, in 1991. The challenges we face are too great for any one nation to bear alone.

'But in this unpredictable environment, our alliance cannot rest on past achievements, or assume that foundational principles simply endure.

'As my Prime Minister said last month: "Ours is an indispensable partnership. We must not disregard everything that has sustained us for the last 80 years. Instead, we must build on it."'

That renewal, Charles said, 'starts with security', adding: 'The United Kingdom recognises that the threats we face demand a transformation in British defence,' in reference to some of the disputes that have dogged Anglo-US relations in recent months.

'That is why our country, in order to be fit for the future, has committed to the biggest sustained increase in defence spending since the Cold War - during part of which, over 50 years ago, I served with immense pride in the Royal Navy, following in the naval footsteps of my father, Prince Philip, Duke of Edinburgh; my grandfather, King George VI; my great-uncle, Lord Mountbatten; and my great-grandfather, King George V.'

Many will see this as a gentle rejoinder to President Trump for his public dismissal of the capabilities of the Royal Navy.

The King also paid tribute to the victims and their families of 9/11, which happened almost 25 years ago.