King Charles led commemorations less than 24 hours after being in hospital for cancer treatment.
The 75-year-old said he wanted to 'lead from the front' at the event after being hospital on Tuesday.
Speaking in Portsmouth on Wednesday, the King delivered an eight-minute address that left tears in Queen Camilla’s eye, reports The Sun.
On medical advice, his appearance was reduced by 45 minutes but he still had the chance to mingle with some of the 21 D-Day heroes invited to the televised event.
It is understood doctors had ordered "compromises" which saw Prince William step in for his father at the international ceremony on Omaha Beach.
Royal sources said: "All events were looked at in consultation with doctors. They were extremely long days and some compromises had to be made."
A source close to William said: "As Prince of Wales, he has a strong sense of wanting to support his father, both at home and overseas."
Charles will attend the King’s Foundation Awards at St James’s Palace, London, next Tuesday.
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Charles addressed an emotional crowd of veterans at the national commemorative event in Ver-sur-Mer in Normandy, France, on Thursday as he spoke of his "profound sense of gratitude" to those who served in 1944.
He appeared emotional during the service, alongside the Queen who wiped her eye as the memories of one D-Day veteran were read to the crowd.
At a Canadian commemorative ceremony on Juno Beach, William thanked those who served "for our freedom", adding: "All of you demonstrated heroism, and determination, that ensured fascism was conquered."
Charles told one veteran he is "doing well" after being asked about his health.
During his speech, the King paid tribute to the "remarkable wartime generation", telling the audience: "It is with the most profound sense of gratitude that we remember them and all who served at that critical time."
He added: "Our ability to learn from their stories at first hand diminishes, but our obligation to remember them, what they stood for and what they achieved for us all can never diminish."
