The King has praised the “resolve and resilience” of farmers “through the challenges of our changing climate”.
Charles wrote the message for a harvest-themed service attended by the Queen at Westminster Abbey on Thursday.
Camilla was all smiles as she arrived at the church which was decorated with floral and fresh food displays.
The King, who did not attend the service, wrote: “It is a time of year to give thanks to the custodians of our land in a tradition that stretches back centuries, yet remains ever relevant.
“Their labour is the foundation of our food production, the heartbeat of rural communities and vital to the protection of the natural landscapes we all treasure so dearly.
“The particular resolve and resilience they have demonstrated through the challenges of our changing climate in recent years deserves our utmost admiration and gratitude – particularly in a year in which the weather has made growing anything so very difficult.
“Our hearts go out to those who are struggling especially, as winter approaches, those with livestock.”
The service was set to include readings by actor Charles Dance and broadcaster Matt Baker, while poet Simon Armitage will read his poem, Bread Of Heaven.
The Queen was then due to meet representatives from charities to hear about their work to tackle food poverty.
