King Charles has extolled the importance of Britain’s “special relationship” with the United States in a speech to US Congress that made pointed reference to the importance of Nato, the defence of Ukraine and the climate crisis.

In a speech that will be read as a veiled plea to Donald Trump to return to the United States’ traditional European alliances and restore his country’s role as a defender of liberal values, Charles said: “America’s words carry weight and meaning, as they have since independence. The actions of this great nation matter even more.”

Charles’s appearance before the joint session of Congress – the first such royal address in 35 years – was billed as the centrepiece of his four-day state visit to the United States, where he and Queen Camilla are taking part in celebrations to mark the 250th anniversary of independence.

Charles praised the historic bond between the two nations, saying: “The alliance that our two nations have built over the centuries – and for which we are profoundly grateful to the American people – is truly unique.”

But in comments that appeared to draw an approving response from Democratic lawmakers, he noted the roots of “the principle that executive power is subject to checks and balances” in one of the UK’s foundational legal documents, Magna Carta.

And he urged “unyielding resolve” in the cause of “Ukraine and her most courageous people” in order to “secure a truly just and lasting peace”.

Speaking from the dais of the House of Representatives before an audience that included lawmakers from both parties and top military officials, he also emphasised the importance of action on the climate crisis.

He made reference to “the natural wonders” of the United States, speaking of what Teddy Roosevelt called “the glorious heritage of this land’s extraordinary natural splendor, on which so much of its prosperity has always depended”.

“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.

“We ignore at our peril the fact that these natural systems, in other words, nature’s own economy, provide the foundation for our prosperity and our national security.”

He also underlined the importance of trade between the two nations at a time when Trump has threatened to impose further tariffs on Britain. “More broadly, we celebrate the $430bn in annual trade that continues to grow, the $1.7tn in mutual investment that fuels that innovation, and the millions of jobs on both sides of the Atlantic supported across both economies.”

He said: “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.”

It was the first such speech by a British royal since Queen Elizabeth II addressed the chamber in 1991, and Charles described the world as having grown “more volatile and more dangerous” since then, making the nations’ alliance more important than ever.

“The challenges we face are too great for any one nation to bear alone,” he said. “In this unpredictable environment, our alliance cannot rest on past achievements, or assume that foundational principles simply endure.”

Charles’ remarks were couched in diplomatic terms, and he remained subtle about the differences of opinion between the US and Prime Minister Keir Starmer’s government, which include Britain’s refusal to join the US and Israel in bombing Iran, and Trump’s complaints about British trade policy.

The address followed an arrival ceremony full of pomp at the White House, in which Trump declared “Americans have no closer friends than the British” amid a military parade and jet flyover.

But in a sign of the sensitivities around the visit, the king’s arrival at the White House took place in private, for fear of a public argument between the two men similar to when the Ukrainian president, Volodymyr Zelenskyy, visited last year.

Congress’s leaders have welcomed the king’s visit, with the Republican Senate majority leader, John Thune, saying in a Tuesday floor speech: “The special relationship between our two nations has been a long and fruitful one, and I think it’s fair to say that over the course of our nation’s history, we’ve had no choicer ally than Great Britain.”

His Democratic counterpart, minority leader Chuck Schumer, leaned in to some of the tensions that have developed between the Trump administration and Starmer’s government over the US’s role in global alliances.

“The visit of King Charles III should serve as a reminder to the president of the United States and to every elected official in this country, friends and allies matter. And most of all, for the security of Europe, Nato matters,” Schumer said.

“I encourage his majesty to impress upon the president the absolute importance of Nato. I hope he’s able to get Trump to listen and understand the stakes.”

In his remarks, the king said that “time and again our two countries have always found ways to come together”, to create “one of the most consequential alliances in human history”. Avoiding mention of Iran, he instead described their military and intelligence cooperation as “hard wired together through relationships measured, not in years, in decades”.

With Trump threatening to shred a trade deal he signed with Starmer and impose a “big tariff” if the UK does not withdraw a digital services tax on US companies, the king called the nations’ longstanding commercial ties “strong foundations on which to continue to build for generations yet unborn.”

He spoke before a Congress that is controlled by Republicans but bitterly divided on a host of issues as lawmakers look ahead to November, when voters will cast ballots in midterm elections that could put the Democrats back in charge.

But the business of the two chambers has occasionally encroached on topics Buckingham Palace might typically wish to avoid. In a near unanimous vote, Congress last year passed a bill requiring the release of files related to the disgraced financier Jeffrey Epstein, just after the king stripped his brother, Andrew Mountbatten-Windsor, of his royal title amid concerns about their relationship. He was later arrested on charges related to his dealings with Epstein while serving as a trade envoy between 2001 and 2011.

The investigative House oversight committee has since asked Mountbatten-Windsor to answer questions about Epstein, who died in 2019 while awaiting trial on sex trafficking charges, with some lawmakers saying he should be issued a subpoena if he does not come voluntarily.