The King gave the Prince of Wales gentle pats on the arm as they prepared to take part in a historical ceremony which honours “promise and service”.
The private moment took place as the father and son, dressed in their crimson satin robes, greeted each other ahead of the pomp and pageantry of the Order of the Bath service at Westminster Abbey.
The special ceremony, which celebrated the 300th anniversary of the order, was also Charles’s first time at the service as the sovereign – and saw William, who was attending for the first time, installed as Great Master of the ancient order.
It is awarded to members of the military or Civil Service for exemplary work, and the Dean of Westminster, Reverend Dr David Hoyle, told guests: “We gather today around the idea that virtue and value are precisely not measured in pounds and pence.
“It is in chivalry, nor is it patronage that binds us together. It’s a shared and ordered belief in promise and service.”
Matters such as “excellence and goodness”, which are worthy of praise, “do not happen by accident or chance”, he reminded those gathered.
He added: “We have to sum them up. We have to remember them, and then, having done that, we have to act – that surely is what the most honourable Order of the Bath should be, what it should celebrate, what it should do.
“Here’s a place to know and name what service of excellence look like.
“Here’s a community that can describe goodness and commit to community and live it out.
“So the finery we see around us is a symbol of a deeper dignity, excellence, honour, and those things will only exist if we name them, it’s a conversation we have to have.”
The lavish ceremony saw Charles and William process through the church with officers, knights and the Dame Grand Cross of the order and take their seats in the Quire.
Charles, who is still undergoing treatment for cancer, and William appeared just two weeks on from his youngest son the Duke of Sussex’s bombshell television interview in which he claimed the King will not speak to him and he does not know how much longer his father has left to live.
The installation ceremony took place in the abbey’s Lady Chapel of King Henry VII, where the William took an oath, followed by five new Knights Grand Cross.
They are Air Chief Marshal Sir Stephen Hillier, Admiral Philip Jones, along with General Sir Mark Carleton-Smith, Lord Young and Sir Tom Scholar.
Charles handed the role, which he held himself for nearly 50 years, to William on St George’s Day last year as part of a wave of prestigious appointments which included ones for the Queen and the Princess of Wales.
During Friday’s ceremony, Charles made an offering of gold and silver at the altar. It was followed by an offering from the newly installed knights whose swords were placed upon the altar.
The Most Honourable Order of the Bath was established by King George I in 1725, although it is believed to originally date back as far as the eighth century.
The title arose from the ritual washing inspired by baptism cleansing, a symbol of spiritual purification, which formed part of the preparations for the conferment of knighthood.
The honour was not given until the candidates had prepared themselves through various rituals designed to purify the inner soul such as fasting, vigils and prayer, and cleansing themselves by bathing.
During Queen Elizabeth II’s reign, the custom of giving an honorary Grand Cross of the Order of the Bath to visiting presidents began, and famous recipients included former US president Ronald Reagan.
The service usually takes place every four years, with the Great Master attending each time and the sovereign every eight.
In April last year, Charles appointed daughter-in-law Kate the first Royal Companion in the history of the Order of the Companions of Honour in recognition of her public service and support of the arts, while Camilla was made Grand Master and First or Principal Dame Grand Cross of the Most Excellent Order of the British Empire.
