The 24-year-old, who lives in Boyn Hill Road, has won three equestrian gold medals at the London 2012 Games with her horse Janeiro, and a postbox in Maidenhead High Street will be transformed by the Royal Mail tomorrow to mark her success in the team open event.
Painting will begin at 10am.
First class stamps featuring Sophie's image will also be on sale in about 500 post offices across the UK tomorrow, and in a further 4,700 branches from Saturday.
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The Met Office has issued a yellow alert for Windsor and Maidenhead, Slough and Wokingham, warning that rain moving eastwards across the south of the country may turn to snow in some places.