The King told a D-Day veteran that he was he was “doing well”, as he and the Queen met heroes of the Normandy landings.
After attending the national commemorative event in Ver-sur-Mer on Thursday, Charles and Camilla visited a marquee where veterans had tea and cakes.
On one table, the King sat beside Ronald Hendrey, 98, of Clacton-on-Sea, Essex, who asked him about his health.
Mr Hendrey said: “He was very nice, he listened to what I was saying and took it in.
“I asked him how he was doing, he told me he is doing well.”
He said he was moved at the number of people who attended the event.
Arthur Oborne, 100, who paid tribute to a friend who saved his life on D-Day, said he became a “bit tearful”.
Mr Oborne spoke at the event, and recalled being shot in the lung three days after arriving on Gold Beach.
His life was saved by his friend Walter Gummerson, who was killed the next day alongside the rest of his unit.
Mr Oborne, from Portishead, Somerset, told the crowd: “I wish I could tell him that I have never taken his sacrifice for granted and will always remember him and our friends.
“So Gummy, thank you my old friend.”
Afterwards he said: “I had no problems talking but it was a bit emotional, I was a bit upset and teary.
“I think everyone did well at the service, I was a bit apprehensive at the start but I wanted to speak.”
Earlier, Gilbert Clarke, 98, who was born in Montego Bay, Jamaica, gave a white flower he received during the event to Camilla.
He said: “I gave it to her because she is a very nice person. She took the time to talk to us, so I wanted to give it.”
Former RAF Sergeant Bernard Morgan was pushed in his wheelchair to the marquee by Prime Minister Rishi Sunak.
Mr Morgan, 100, from Crewe, was a code-breaker who landed on Gold Beach in the early evening of D-Day and saw the grim sight of drowned servicemen.
His daughter, Sheila, said: “I was surprised to see dad being pushed by the Prime Minister, it was quite the moment.”