A catwalk event will be put on at the racecourse's Bessborough Restaurant which can hold up to 1,750 people over the five-day race meet.
The show, the third since the 2007 premiere, will also be broadcast on big screens around the course to around 300,000 visitors. It will also feature collections from Stephen Jones and Luella.
Vivienne Westwood said: "Royal Ascot is a British institution for showing off clothes and I like the idea of a group of people gathered together."
A spokeswoman for Royal Ascot added: "The reaction to the shows have been fantastic. This is a once in a lifetime opportunity for public to see big designer shows as fashion weeks are for trade only.
"The designers in the 2009 show do not show at London fashion week preferring Milan, New York and Paris where the events consist of rushing from one venue to the next and where shows are often delayed."
Ascot is currently .
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