The 39-year-old actress turned heads in the knee-length lace dress, which was cinched in at the waist with a black silk sash.
She teamed her outfit with patent heels, a clutch and the indispensible OTT Ascot hat - a black and white floral one to complement her dress.
Do you think she was dressed smartly enough for the event, which included such big names as Queen Elizabeth II, Princess Beatrice, and Princess Eugenie? Tell us below!
Text: Sify; Images: AP