The King was joined by several foreign monarchs as they came together to remember and mourn the millions of Holocaust victims at a service in Poland to mark the 80th anniversary of the liberation of Auschwitz.
As Charles, 76, took his seat at the ceremony in front of the gates of the former Nazi concentration camp, he was reunited with King Frederik and Queen Mary of Denmark.
The monarchs shared a warm greeting and interaction, with Frederik, 56, affectionately patting Charles on his shoulder.
Watch their exchange below…
The then Crown Prince Frederik and Crown Princess Mary witnessed the King's coronation in London in May 2023, with Charles later congratulating the Danish royal couple on the day of Frederik's accession in January 2024.
In his letter to the new Danish king and queen, Charles wrote: "My wife joins me in writing to convey our very best wishes on the day of your accession to the throne of the Kingdom of Denmark.
"I look forward to working with you on ensuring that the enduring bond between our countries, and our families, remains strong, and to working together with you on issues which matter so much for our countries and the wider world.
"I pay tribute to the many years of service of your mother, Her Majesty Queen Margrethe II, and remember fondly the frequent visits between our countries, including our visit to Copenhagen and Elsinore in 2012.
"I was delighted that Your Majesties were able to attend our coronation last year and I much look forward to future opportunities to celebrate the close connection that unites our countries and our families."
The British and Danish royals share close ties – Queen Margrethe and the late Queen Elizabeth II were third cousins, with both monarchs descending from Queen Victoria and King Christian IX of Denmark.
King Frederik and Queen Mary have also met with the Prince and Princess of Wales on several occasions, with Kate joining Mary in Copenhagen during an official visit in 2022.
"Sacred duty"
The King described his historic visit to Auschwitz as "something that I will never forget" and said remembering the "horrors" of the concentration camp was a "sacred duty".
Charles appeared visibly moved as guests listened to harrowing survivor stories during the ceremony in Poland.
Candles were left by heads of state and government after the survivors had placed their flickering tributes, and the King placed his light with both hands, before taking a step back and bowing his head.
He later left a wreath at the Death Wall, a reconstruction of the site where several thousand people, mainly Polish political prisoners, were executed and he also paused under the infamous iron gates that have come to symbolise the terror of the Nazi regime, bearing the motto Arbeit macht frei – Work Sets You Free.