Charles and Queen Camilla are on a four-day state visit to the US, steering clear of the political rifts between Trump and British Prime Minister Keir Starmer while underscoring the deep ties forged between Britain and its former colony over the 250 years since independence.
The King's address on Tuesday, scheduled to begin at 3pm EDT (5am on Wednesday AEST), will be only the second by a British sovereign to the US Congress, after his mother Queen Elizabeth II spoke to both houses in 1991.
It is a central event of the state visit - the most high-profile of Charles' reign - and will be followed by a state dinner on Tuesday night.
The King's speech was expected to last about 20 minutes, a palace source said, and touch on issues including NATO, the Middle East and Ukraine.
While written on the advice of the British government, much of the language and tone came from Charles himself, the source said.
Its central message will focus on challenges facing both nations, stressing that the two countries can promote international security and prosperity by defending their shared values.
Despite periodic differences, the King will say, "Time and again, our two countries have always found ways to come together," and will describe the partnership as "one of the greatest alliances in human history", the source said.
Trump, a vocal admirer of the British royal family who often calls Charles a "great man," has nonetheless clashed with Starmer's government.
Starmer hopes the visit will shore up the trans-Atlantic relationship, which has been strained in recent months.
The long-planned visit has unfolded against tensions over the US-Israeli war on Iran, after Trump criticised Britain for declining to support the offensive.
Although Trump has eased his criticism in recent days, an internal Pentagon email outlining how Washington might review its position on Britain's claim to the Falkland Islands has raised concern across the Atlantic.
The visit began on Monday when Charles and Camilla met Trump and first lady Melania Trump for a private tea at the White House, followed by a garden party at the British ambassador's residence, where they mingled with media leaders, Washington socialites and US officials.
On Wednesday, the royals will be in New York City to commemorate those killed in the attacks of September 11, 2001, and the trip concludes in Virginia on Thursday with the king meeting people involved in conservation, reflecting his long-standing environmental advocacy.
