Security appeared not to recognize the Princess Royal's daughter, who was accompanied by her husband Mike Tindall, 34, on the opening day of the racing event.
Prevented from crossing the parade ring after a race, Zara, 31, was seen in an animated discussion with the guard, as she tried to access the enclosure.
She was eventually let her through, and was seen laughing off the incident with former England rugby caption Mike, who she married in 2011, and Chanelle, the wife of jockey AP McCoy.
Zara looked elegant in a cream coat and a purple feather hat as she watched the action at the races, along with 60,000 others, who descended on the festival despite the weather.
Following her appearance at the London Olympic Games, the silver medallist was back in the saddle at the weekend as she took part in the opening competition of the 2013 British Eventing season at Isleham.