The King said he is "greatly encouraged" after interrupting his summer holiday with Queen Camilla to mark Ukraine's Independence Day.
Charles, 77, and his wife, 79, sent "sincere and special wishes" to Ukrainian president Volodymyr Zelensky, who shared the message on social media on Tuesday and called their words "deeply moving and exceptionally meaningful".
In a personal message, released through the British Embassy in Kyiv, the King wrote: "Both my wife and I are so pleased to be able to extend to you and to the people of Ukraine our heartfelt congratulations on the occasion of your Independence Day."
After reflecting on Ukraine's "proud history, rich culture and indomitable national spirit", the monarch added: "I remain greatly encouraged by the enduring friendship between our nations and by the close bonds which unite our peoples.
"My wife and I send our sincere and special wishes to you and to all Ukrainians for the possibility of peace, security, and prosperity in the year ahead."
King Charles's support for Zelensky
The King met Zelensky at Buckingham Palace back in March, ahead of signing a new defence agreement with the UK.
Charles also spoke of the importance of Ukraine in his speech to Congress during the US State Visit in May.
Although the monarch must remain a neutral political figure, he often greets key prime ministers, presidents, leaders and dignitaries to build relationships with other countries and use diplomatic soft power.
© Getty ImagesZelensky has been welcomed at various royal residences over the years, flying into Britain to meet the King for a top-secret first meeting in 2023.
Speaking about His Majesty after one of the meetings in 2025, Zelensky said: "I'm very grateful for this meeting and for how His Majesty helps Ukraine and supports us. He's met our warriors here, being trained in the UK, and we're very thankful to the Royal Family for their support."
It comes as Prime Minister Andy Burnham made his inaugural visit overseas in the role to visit Kyiv, co-chairing a Coalition of the Willing meeting with Germany's Chancellor Friedrich Merz and France's President Emmanuel Macron joining by video link.
King Charles's stay at Balmoral with the royal family
The King and Queen are currently enjoying their summer break in Balmoral away from the spotlight with other members of the royal family.
After receiving his official welcome to the Aberdeenshire Castle last week, Charles and Camilla joined the Waleses for a church service at Crathie Kirk.
© Getty ImagesThe King and Queen travelled in the first car, while William was driving the second car – which also carried Kate and the couple’s three children.
The Princess Royal and her husband Sir Tim Laurence were in another of the cars, and The Duke of Edinburgh and his daughter Lady Louise were in another vehicle.
The visit came days after news of Prince Harry and Meghan Markle's imminent return to the UK emerged.
