The King has said that the US lies at “the heart” of Nato in an apparent rebuke to President Donald Trump’s threats to leave the defensive alliance.

In a historic speech to the US Congress, King Charles said the UK government had committed to the biggest sustained increase in defence spending since the Cold War, in an attempt to address the president’s claims that the US has propped up Europe and Nato over military power.

He also referenced the 11 September 2001 attacks on the US, when Article 5 of Nato was invoked for the first time, to hint that Ukraine still needed defending.

“That same, unyielding resolve is needed for the defence of Ukraine and her most courageous people – in order to secure a truly just and lasting peace,” he said, to applause.

“From the depths of the Atlantic to the disastrously melting ice caps of the Arctic, the commitment and expertise of the United States Armed Forces and its allies lie at the heart of Nato, pledged to each other’s defence, protecting our citizens and interests, keeping North Americans and Europeans safe from our common adversaries,” he said.

“Our defence, intelligence and security ties are hardwired together through relationships measured not in years, but in decades.”

The King was addressing the politicians during his visit with Queen Camilla to celebrate the 250th anniversary of the signing of America’s Declaration of Independence.

As he sought to heal a rift between the two countries’ leaders, he went out of his way to stress the historic ties between the two nations, saying it was “one of the greatest alliances in human history”, and that the UK-US partnership “is more important today than it has ever been”.

Referencing the US becoming independent in 1776, he said: “The very principle on which your Congress was founded – no taxation without representation – was at once a fundamental disagreement between us, and at the same time a shared democratic value which you inherited from us. Ours is a partnership born out of dispute, but no less strong for it.”

The King also made reference to Saturday’s shooting at the White House Correspondents’ Dinner. The event was thrown into chaos after the suspected lone gunman, identified as Cole Tomas Allen, allegedly rushed past security and fired at least one gunshot. He had claimed to be targeting members of the Trump administration.

“Whatever our differences, whatever disagreements we may have, we stand united in our commitment to uphold democracy, to protect all our people from harm, and to salute the courage of those who daily risk their lives in the service of our countries,” the King said.

He insisted the story of the UK and US over the past 250 years was one of “reconciliation and renewal”, adding: “I pray with all my heart that our Alliance will continue to defend our shared values, with our partners in Europe and the Commonwealth, and across the world, and that we ignore the clarion calls to become ever more inward-looking.”

The UK and the US always “find ways to come together”, the King told the joint meeting of Congress.

In an effort to improve strained relations between the UK and US, he expressed “the highest regard and friendship of the British people to the people of the United States’ in this landmark anniversary year”.

The King also reminded his audience of the climate crisis, saying: “As we look toward the next 250 years, we must also reflect on our shared responsibility to safeguard Nature, our most precious and irreplaceable asset.”

He added: “Yet even as we celebrate the beauty that surrounds us, our generation must decide how to address the collapse of critical natural systems, which threatens far more than the harmony and essential diversity of Nature.”

Mr Trump has in recent months repeatedly attacked prime minister Sir Keir Starmer over the government’s refusal to join his war on Iran and other domestic policies, such as migration and renewable energy.

Earlier, the US president said the US had “no closer friends than the British”.

Speaking from a podium on the White House South Lawn on the second day of the King’s historic visit, he said: “In the centuries since we won our independence, Americans have had no closer friends than the British.”

And he paid tribute to the “essence” of the “special relationship” following the Second World War, saying: “That understanding of our nation’s unique bond and role in history is the essence of our special relationship and we hope it will always remain that way.”

Mr Trump, who has criticised Sir Keir for being “no Winston Churchill” over the UK’s response to US calls for support in the Iran conflict, pointedly referred several times to wartime leader Sir Winston in his speech, calling him “Britain’s greatest prime minister”.