Trumps ready for tea with the Queen

US President Donald Trump is expected to adopt a softer tone than the one he set earlier this week at a NATO summit when he takes tea with Britain's Queen Elizabeth II.

Trump and his wife Melania are due to meet the 92-year-old monarch at Windsor Castle on Friday and both the imposing setting and Elizabeth being one of the most admired women in the world is expected to temper the US president's iconaclastic ways.

The Queen has met every US president since Dwight Eisenhower with the exception of Lyndon Johnson, who never visited Britain while in office.

There are rules of etiquette for encounters with the Queen, although they are less rigid than many believe.

Chief among them: Don't get too affectionate with the queen, who does not expect to be hugged or kissed by guests.

Etiquette will require the Trumps to wait until Elizabeth offers her hand, then to shake it politely and move on.

Neither will be likely to bow or curtsy when they meet the Queen, said Hugo Vickers, an author who has long chronicled the British royals.

"That wouldn't be required from a head of state or the wife of a head of state," he said.

"He would be wise not to attempt to kiss her, and I don't expect for a moment that he will."

Trump has told British tabloid The Sun that he's not nervous about meeting the Queen, who he called "a tremendous woman".

"I really look forward to meeting her. I think she represents her country so well," he told The Sun.

There are conventions on how to address the Queen. The Trumps will be expected to call her "Your Majesty" upon meeting her and afterward call her "Ma'am."

But the Queen wasn't upset when South African President Nelson Mandela called her Elizabeth. Nor was she miffed when Michelle Obama put her arm around the queen briefly.

Elizabeth, the longest reigning monarch in British history, has shown herself to be unflappable when the unexpected occurs. She is not known to show anger in public, and the only times most people have seen her show unbridled glee is when one of her racehorses performs extremely well in a competition.