Sophie marked the special occasion with her ten-year-old daughter, who is a Brownie herself, by throwing a party in the palace garden complete with games, tennis and a bespoke birthday cake.
Young Louise, who was dressed in her yellow and brown Brownie uniform, looked thrilled as she joined other girls playing giant games of Jenga and Connect Four on the lawn.
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Sophie was dressed in loose-fitting patterned trousers and a black top which she had worn earlier in the day to open the Royal Kitchen Garden at Hampton Court Palace.
The Countess proudly sported a gold badge bearing the Brownie logo which symbolises her role as president of Girlguiding.
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Sophie is dedicated to her duties with the 100-year-old organisation and when the centenary year began in January she paid a visit to 5th Frimley Brownies in Surrey.
Prince Edward's wife spent time chatting to excited youngsters in the group and taking part in their activities before taking a break to enjoy a cupcake.
During her visit, the mother-of-two was shown the commemorative badges the Brownies had received to mark Prince William and Kate Middleton's royal wedding and the birth of their son, Prince George.
Sophie's outing came shortly after it was revealed that her daughter Louise had undergone successful eye surgery to correct her exotropia – a condition that turns the eye outwards.