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The royal, who was joined by her husband Prince Edward and their two children Lady Louise Windsor and James, Viscount Severn, sported a green and grey plaid coat with a magenta velvet collar and buttons.
Under her chic coat, the 48-year-old wore a belted purple dress, which was visible when she and her family watched the races from the course's balcony.
The mother-of-two accessorised her look with tights, light brown boots and matching diamond earrings and necklace.
Smiling and looking relaxed, Countess Sophie joked and laughed with her daughter Louise, who was dressed in a beige trench coat, similar to her father's.
James, who turned six on Tuesday, looked smart in a blue tartan shirt under a navy jumper.
The family's appearance at Ascot came just a day after Peter and Autumn Phillips took their daughters to the races.
Princess Anne's son Peter, 36, and Autumn, 35, were seen playing with Savannah, two, and one-year-old Isla, who were cosily wrapped up against the cold.
Dressed in a tan coloured coat and sporting a blue hat bearing a cat's face and ears, Isla happily watched proceedings from her doting father's arms.
Savannah, who was the Queen's first great grandchild, walked hand-in-hand with her Canadian-born mother.
The British royals are soon expected to gather together at Sandringham, where they will spend Christmas.