Prince William has praised brothers Jordan and Cian Adams as "inspiring" after sending them a personal message of encouragement during their gruelling 33 marathons in 33 days challenge across Ireland.
Both siblings from Redditch, Worcestershire, carry a rare genetic mutation linked to early-onset frontotemporal dementia, meaning they face the likelihood of developing the condition in their 40s.
Their extraordinary fundraising effort has now generated more than €1.3 million, equivalent to nearly £1.2 million, benefiting the Alzheimer Society of Ireland and their own charity, the FTD Brothers Foundation.
Jordan, aged 30, is completing the marathon distances while his 25-year-old brother Cian, who works as a physiotherapist, provides support throughout the journey.
Their father Glenn delivered the royal correspondence to them on Sunday morning as they reached day 14 of their endeavour.
The challenge holds profound personal significance for the brothers, whose mother Geraldine succumbed to FTD at just 52 years of age.
Jordan said his mother would have been "incredibly proud" of receiving the letter from William.
The ambitious undertaking began on April 26 when Jordan completed the London Marathon whilst carrying a 25kg fridge strapped to his back.
Immediately afterwards, the pair crossed to Ireland where Jordan is tackling a marathon distance daily across all 32 counties on the island.
The family has been devastated by the hereditary condition, having lost 12 Irish relatives to FTD over the years.
Jordan received his own diagnosis eight years ago, and has since dedicated himself to raising awareness and funds for dementia research alongside his younger brother.
In his letter, William wrote: "Taking on such a demanding challenge, to honour your mother's memory and raise awareness of frontotemporal dementia, speaks to your remarkable strength."
The heir to the throne added: "It takes great courage and generosity to turn such personal adversity into hope for others."
He praised the brothers for helping to transform public understanding of dementia through sharing their experiences openly, telling them they are providing "countless families strength, comfort, and a sense that they are not alone".
William expressed hope that both men feel proud of their achievements thus far and wished them success for the remainder of their journey.
The timing of the letter proved particularly moving, arriving as the brothers passed through County Leitrim, a region where many of their 12 Irish relatives who died from FTD had lived.
Jordan described the royal recognition as "absolutely incredible and feels incredibly surreal", adding "I don't think it'll sink in for a long time".
Reflecting on his diagnosis eight years ago, he said: "I could never have imagined that we could have created the impact that we have or reached the corners of the world that we have, by sharing our story and trying to be a voice of hope for millions of families."
Cian emphasised the significance of attracting attention from influential figures, noting that dementia research remains considerably underfunded compared to other diseases.
He said: "It's people in these positions that matter, who are starting to get wind of our story and the importance of putting dementia on the map."
The brothers are due to complete their challenge in Dublin on May 28, with Jordan saying the letter "will definitely keep us going" until the finish line.
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