The meeting, a weeklong series at the Ascot racecourse near Windsor, marks its 301st anniversary this June. Queen Anne began the tradition in 1711 with the Queen's Plate, a race for the then-fabulous prize of 100 guineas.
Nick Smith, a spokesman for the Ascot race track, said efforts to lure more attendees with a more relaxed dress code had backfired, The Daily Telegraph reported. He said some race-goers had relaxed their attire far too much.
Smith said the new code takes effect this Saturday with the first race meeting of the season. That gives stewards time to adjust to the new era before Royal Ascot.
During Royal Ascot, Queen Elizabeth II or other members of her family are in the royal box every day, arriving by carriage, and opening the day with the royal procession and the raising of the queen's standard. Dress rules in the Royal Enclosure are very strict, with men in morning coats and top hats, and women in hats and dresses banned from showing their shoulders, midriffs or too much leg.