Bardot necklines, named after actress Brigitte Bardot who built the off-the-shoulder appear famous, ought to have prohibited from the Royal Enclosure at Ascot.
Fascinators are also banned and for men, plain black and grey top hats are the only style permitted this summer.
Last year’s addition of jumpsuits to the list of acceptable outfits remains but they must be full length.
Organisers said: “Ascot continues to recognise key trends in the ever-evolving world of fashion.”
The event’s official dress style guide likewise attains socks a requirement for gentlemen.