The US trip concludes in Virginia on Thursday with the king meeting those involved in conservation work, reflecting his long-standing environmental advocacy.

He will then travel to Bermuda.

King Charles III told Congress earlier Tuesday that the same “unyielding resolve” seen after the September 11, 2001, attacks on the US was “needed for the defence of Ukraine and her most courageous people … to secure a truly just and lasting peace.”

The king’s visit comes at a time of tensions between the two long-time allies over the war with Iran.

In a speech to Congress earlier, Charles made no direct mention of the Iran war, but referred to NATO, highlighted the importance of continued US help for Ukraine in its war with Russia, and the dangers of isolationism.

Both Britain and the United States have maintained over the years that Tehran should not develop nuclear weapons.

Charles began his speech by paying tribute to the security services “for their swift actions on Saturday evening in preventing further injury” at the shooting incident at the White House Correspondents’ dinner, when Donald Trump was hustled out of the room.

“As your own response demonstrates, what used to be called in the last war in the United Kingdom: Keep calm and carry on,” he said.

He also drew laughs with a quip about the White House renovations.

“We British, of course, made our own small attempt at real estate redevelopment of the White House in 1814,” he said.

He acknowledged that the two countries have had their “moments of difficulty even in more recent history”, but stressed that “it is not hard to see how important the relationship remains in matters both seen and unseen.”

In the 20th century, “American leadership has helped rebuild a shattered continent, playing a decisive role as a defender of freedom in Europe,” the king said.

“We, and I shall never forget that. Not least, as freedom is again under attack following Russia’s invasion of Ukraine.”