Ruby Wax , 72, spotted the royals on the Isle of Iona, in Scotland, after they had travelled there to meet the local community and learn about their way of life.
William and Kate's Royal Foundation will also announce a community project as the couple meet locals while remembering their 2011 wedding in Westminster Abbey.
She may be known for her classic country apparel and iconic riding boots. But Catherine has proved she can do both haute couture and effortless casual looks.
Ahead of today's tense match in Cardiff, Prince William and Princess Kate met injured players supported by the Welsh Rugby Charitable Trust - one of the Prince's patronages.
King Charles is being joined by Queen Camilla, the Prince and Princess of Wales and a 2,000-strong congregation for the annual Commonwealth Day service at Westminster Abbey today.