AARON CHOWN/POOL/AFP via Getty Prince William and Prince Harry
The Prince of Wales, 40, and the Duke of Sussex, 38, jointly signed a letter praising the work of a charity they both support, and honor a family "we all love and adore."
Their message came at the annual carol service raising money for the Henry van Straubenzee Memorial Fund, a non-profit that was set up by the parents, Alex and Claire, and brothers of their late friend, Henry van Straubenzee.
The charity supports schools and helps construct, restore and improve buildings, enabling children to have better and properly-equipped classrooms and facilities like restrooms. When he died in a car crash in 2002, Henry had been due to travel to Uganda to work in a school during his gap year.
Kirsty O'Connor/AP/Shutterstock Prince William, Kate Middleton, Prince Harry and Meghan Markle at Windsor Castle
William and Harry's jointly-signed letter was carried in the order of service at Thursday's carol service, which was attended by close friends, including Princess Kate's sister and brother, Pippa and James, and their spouses, at St. Luke's Church, in Sydney Street, Chelsea.
Indigo/Getty Prince Harry and Prince William in happier times in 2011
Together, the princes wrote, "We believe everyone here tonight will agree that what Claire and Alex have achieved since 2009 ... is nothing short of extraordinary. As you gather under this roof tonight, the charity is supporting 51 nursery, primary and secondary schools in rural Uganda, where 1,700 projects have been completed and over 35,000 children, each year, are benefitting by receiving education they wouldn't have otherwise had."
"For those of us who knew Henners, we will always miss his mischievous charm and hilarious sense of humor," they continued.
And though the brothers noted that the charity will be gradually wound down, the letter said, however, that "Henry's legacy will live on through the incredible achievements of what his mum and dad have accomplished over the years."
Wishing everyone an "enjoyable evening and very Happy Christmas," the letter concluded: "As long as funds permit, they will continue to monitor their schools, enduring that the children of south east Uganda receive that vital education, providing the opportunity to escape poverty as they grow up, and hopefully that of their own children in the future. Thank you for your continued support in making this possible, and for the everlasting support of the van family who we all love and adore."
ANGELA WEISS/AFP Kate Middleton and Prince William
The princes couldn't be at the carol service in-person for different reasons.
William landed back in Britain on Saturday morning alongside Kate, 40, after their three-day trip to Boston, which ended with the triumphant Earthshot Prize ceremony at the MGM Grand at Fenway.
Harry, meanwhile, was in California, where he lives with Meghan, 41, and their children, son Archie, 3, and daughter Lilibet Diana, 1.