Charles could not resist nibbling one of the sweet treats when he staged a garden party at Clarence House to celebrate the 21st anniversary of his Duchy Originals brand.
Since going on sale in 1992, consumers have enjoyed 70 million oaten biscuits - the first Duchy product and still one of its most popular items.
Now managed by Waitrose, the brand has expanded to 230 products, featuring everything from bacon and jam to beer and ciders, and has generated more than £11 million for charitable cause over the decades.
When Charles came across a stall manned by Michael Cox, a senior account manager from Scottish firm Walkers Shortbread, he took a bite of an oaten biscuit made by the firm.
Jim Walker, the company's joint managing director, was watching and later recalled how, during the biscuit's development, they had sent regular samples to Charles, who wanted something "wholesome" and not "too sweet".
The Duchess of Cornwall was also feeling peckish and could not resist sampling some of the Duchy products on display.
Charles and Camilla arrived at the reception after attending a Requiem Mass at Brentwood Cathedral in Essex for the Prince's close friend, Hugh van Cutsem, who died last week.
They may have missed lunch because after eating a tiny open sausage sandwich, the Duchess joked: "I'm making up for lost time."
Charles founded Duchy Originals in 1990 to promote sustainable produce and champion organic foods.
Among the guests invited to celebrate the brand were all elements of the chain that helped to make the goods a success, from farmers and customers to charities that have received donations from the funds raised.
In a speech, the Prince told them that the oaten biscuit "has endured as one of the brand's most popular products especially, I'm glad to say, in the ever- growing international market".
He went on: "Thank you to all of you who are here today who have played such an important part in helping to create a brand, at the end of the day, that really does take the biscuit."
Charles added: "The brand represents the very best of sustainably sourced and organic British products.
"Right from the start preserving our heritage, upholding traditional skills and supporting our local communities were a fundamental importance to the brand.
"If that was important 21 years ago, I happen to believe it is even more so today. With accelerating climate change, exponentially rising costs of fuel and fertiliser and an unsustainably growing population, we have to find ways to live more sustainably."