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 included readings by actor Charles Dance and broadcaster Matt Baker, while poet Simon Armitage read his poem, Bread Of Heaven.
Before leaving, the Queen met charity representatives working to combat food poverty, before chatting with greengrocer Peter Thomas, 72, from Loughton, Essex, about his colourful fruit and vegetable display outside the church.
Mr Thomas said he arrived in the early hours to set up the display, which took about three hours.
Camilla then admired a 1957 tractor shown by farmer Peter Drinkwater, 65, from Ebrington, in the Cotswolds.
When he mentioned his village, the Queen smiled and said, “oh, not very far from me”, referring to her residence at Highgrove.
Mr Drinkwater said she spoke fondly of the tractor, adding that she had one herself.
