The King raised his eyes heavenwards in mock embarrassment as he was serenaded at a tea party on the eve of his 75th birthday.
Charles joined a host of fellow 75-year-olds for the celebration, which included live music from a local rock choir and afternoon tea, in the Orchard Room at his Highgrove home.
He arrived at the party just as the choir was playing Hallelujah by Leonard Cohen, one of his favoured artists, whose “extraordinary” lyrics he once described as “very moving”.
The King, who turns 75 on Tuesday, was immediately presented with a cup of tea before joining guests for a natter.
Later, there was a rousing version of Happy Birthday, before the King waved a knife aloft and said: “Thank you very much.”
He then asked where he should cut into the three-tier cake, topped with a gold crown and decorated with a gold number 75.
Charles used both hands to cut a hearty slice from the lower tier, prompting a rousing cheer and another celebratory wave of the knife.
He then peered inside the Victoria sponge cake. “It’s always fun to have a look,” he said, before urging guests to dig in.
“Who can I donate it to? Can we get rid of it? We need to give you doggie bags,” he laughed.
The party guests included Jay Blades, The Repair Shop presenter, as well as sitters from the Windrush: Portraits of a Pioneering Generation project, commissioned by the King to mark the 75th anniversary of the arrival of HMT Empire Windrush to the UK.
Blades, an ambassador for the King’s Foundation, paid tribute to Charles, saying: “He’s the new King but he’s also about community, he’s always been about community, always been forward thinking about what we’re doing about the environment but also including community members.
“So to have this group of people here, that’s just unbelievable – that’s part of him, that’s what he does.”
Also among the guests was Michelin-starred chef Raymond Blanc, 73, who hailed the monarch a “crusader”.
Mr Blanc, who has recently become an ambassador for the King’s Foundation, said: “He has been campaigning for 50 years on environmental and social issues that are vital for our communities and our planet. What he has done is extraordinary.
“He is a crusader and I hope the Government will give him a little bit more room to express himself.”
He added: “I am a French republican and I have the utmost respect for what he has done.”
Other guests, all of whom were born in 1948, applied to attend via a ballot and were invited in recognition of their contributions to their local communities.
Edna Henry, one of those who sat for portraits of the Windrush generation, congratulated Charles on turning 75.
“So are a lot of other people here, which is far more important,” the King said modestly.
Among the guests was Peter Newell, from Heywood in Wiltshire, who was with his wife Marilyn, both turning 75 this year.
Mr Newell, who formerly worked for Southern Electricity, helped in the construction of the Orchard Room back in 1999.
“I was actually in charge of getting the electricity to it,” he said. “So I spent a lot of time here. I first came here in 1980.”
He enjoyed his dealings with the then Prince Charles, who was hands on with the project.
“He was very good,” he said, adding how they agreed today they would both try to make it to 100.
“I said to him I’m hoping he is going to give me a card for 100. He said: ‘I’ll do my best.’”
An original composition was created for the occasion by 14-year-old Bill Goulding, a local gardening enthusiast who regularly visits Highgrove Gardens.
The King has chosen to use his birthday to highlight causes close to his heart.
Charles, who will still receive his daily red boxes full of policy papers, Foreign Office messages, letters and other state papers on his big day, will also spend his anniversary carrying out public engagements.
First, he will visit a surplus food distribution centre with the Queen, 76, to officially launch the Coronation Food Project, which aims to tackle food poverty by redistributing produce destined for landfills.
Then later, he will host a reception at Buckingham Palace for 400 nurses and midwives, as part of this year’s NHS 75 celebrations, where he will be treated to a surprise birthday song by the NHS choir.
Gun salutes will sound in Green Park and the Tower of London in the capital, and across the UK.
On Tuesday evening, Charles’s nearest and dearest are expected to gather at Clarence House for a birthday dinner in his honour.
Camilla once admitted that her husband was particularly hard to buy gifts for.
“I will tell you that he is the most difficult person in the world to buy a present for … So he likes to make a list of things that he wants so you get it exactly right,” she said.
He likes “a cake and a bit of a sing-song”, she acknowledged, but said it was difficult to get her workaholic husband to take a break.
Last week, at an engagement in New Malden, Kingson upon Thames, the King joked that he was finding it hard to forget his fast-approaching birthday.
John Azah, CEO of Kingston Race and Equalities Council, wished the King a happy birthday for next week. “How did you know?” came the reply. “I can’t escape.”
When he turned 70 in 2018, Charles referred to his birthday as “alarming” and joked he had “all the scars that go with” his age.