Joined by the Queen at his side, the King visited a food distribution hub in Oxfordshire to kick-start his Coronation Food Project - which aims to support charities feeding the nation with unwanted food.

While at a tour of the South Oxfordshire Food and Education Alliance, the King walked over to the delivery vans and a small group of people nearby began singing Happy Birthday.

After the impromptu celebration, he greeted Katy Anne Perugia who led the rendition. She said of the meeting: "The King said to us 'how did you know it was my birthday' and we all laughed."

Before leaving, the King also met senior representatives from leading supermarkets and budget food retailers who have signed up to the Coronation Food Project pledge to distribute more surplus food.

The King smiled as he was serenaded once again with those invited singing Happy Birthday.

The head of state is the cover star of the latest edition of the Big Issue.

Writing in the magazine, he said: "Food need is as real and urgent a problem as food waste - and if a way could be found to bridge the gap between them, then it would address two problems in one.

"It is my great hope that this Coronation Food Project will find practical ways to do just that - rescuing more surplus food, and distributing it to those who need it most."

The King was seen handing seller Kelvin, 61, £10 for the magazine costing £4 at the end of the event in Oxfordshire.

The vendor, who has been homeless for periods of his life since he was a teenager, said of the project: "I think it goes to show he cares, he's reaching out to the general public."

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Meanwhile, across London, gun salutes were fired to mark the King's birthday, with 41 volleys fired at noon by the King's Troop Royal Horse Artillery from Green Park, and 62 volleys fired by the Honourable Artillery Company at the Tower of London - an extra 21 for the City of London.

London's Piccadilly Lights were also lit up with a birthday message for the King, with the tourist attraction displaying a quote from his Big Issue article.

On Monday, the King was joined in the grounds of his Highgrove home in Gloucestershire by celebrity chef Raymond Blanc, The Repair Shop host Jay Blades and singer Leee John from the 1980s group Imagination in an early celebration.

A local choir sang at the end of the tea party, while the King was cheered and applauded after cutting a slice of his three-tiered Victoria sponge cake.