King Charles is expected to allude to recent strains between the UK and US in a rare address by a monarch to the US Congress as he will underline that “time and again our two countries have always found ways to come together”.

The king’s remarks in a speech to both houses on Tuesday will come after Donald Trump has threatened to tear up a trade deal signed by the UK and US, mocked the Royal Navy and insulted the UK prime minister.

Donald Trump’s anger with the UK and Keir Starmer is largely driven by the latter’s refusal to take part in the US and Israeli offensive against Iran, which continues to destabilise the global economy.

Charles and Camilla are in Washington on a four-day state visit to the US to mark the 250th anniversary of American independence from the UK.

In his speech, the king is expected to reflect that while the UK and US have not always agreed on all matters over the past 250 years, the foundations of their “democratic, legal and social traditions” – stretching all the way back to Magna Carta – are such that “time and again our two countries have always found ways to come together”.

As Trump threatens to impose “a big tariff” on the UK if it does not drop its digital services tax on US technology companies, Charles will reference the importance of trade and technology deals that will play a vital part in the two countries’ continued economic partnership.

The start of the trip – Charles’s first to America as king – has been overshadowed by an assassination attempt against Trump at a Washington media dinner on Saturday night.

The monarch is expected to make brief reference to the attack and offer “the highest regard and friendship of the British people to the people of the United States” on the 250th anniversary of American independence.

It is only the second time a British monarch has addressed a joint meeting of Congress, after Queen Elizabeth II did so in 1991.

The king will point out how the US and UK have stood together through moments that have defined their shared history, and that “our defence, intelligence and security ties are measured not in years but in decades”.

As the war on Iran drives up the cost of living around the world, an effect that critics have called “Trumpflation”, Charles is expected to say that by defending and reaffirming their common values and ideals, the US and UK can promote security and prosperity now and in the future for the benefit of the world.

He will hail America’s natural wonders and stress how important it is to defend and protect “nature’s own economy”.

Referring to his faith, he will express his belief that in the hearts of the two countries lies “a generosity of spirit and a duty to foster compassion, to promote peace, to deepen mutual understanding and to value people of all faiths and none”.

He is expected to conclude by noting that the story of both nations over the last 250 years is one of “reconciliation and renewal”, giving rise to “one of the greatest alliances in human history”, which he hopes will continue to safeguard their shared values long into the future.

The speech, which has been written on the advice of the government, is expected to last 20 minutes.