The fashion stakes are always high at Royal Ascot but organisers are now raising the bar by banning fascinators in the royal enclosure.
The decision is part of a move to tighten and clarify the dress code at the annual summer event and comes amid criticism of sartorial standards which have been more loosely enforced in recent years.
Thousands of visitors who flock to the less formal grandstand enclosure during the week-long meet in June will also be obliged to adhere to strict new guidelines.
Organisers have defended the changes and insist the revised dress code is designed to restore formality rather than encourage "elitism".
Ascot spokesman Nick Smith said: "It is probably fair to say that the dress code hasn't necessarily been enforced quite as rigorously as we might have liked.
"It is stretching a point to say standards have collapsed but there is no doubt that our customers would like to get back to a situation where it is universally acknowledged that this is a formal occasion and not an occasion where you might dress as you would at a nightclub."
In the royal enclosure, this means fascinators - which are often favoured by the Duchess of Cambridge - are no longer deemed acceptable. The new dress code states: "Hats should be worn; a headpiece which has a base of four inches (10cm) or more in diameter is acceptable as an alternative to a hat."
Women will also be expected to wear skirts or dresses of "modest length" which fall just above the knee or longer. This clarifies previous guidance which stated miniskirts were "considered unsuitable". For men, a waistcoat and tie are now compulsory in this area of the course and cravats will not be allowed. Black shoes must also be worn with morning dress.
In the grandstand, which is open to the public and subject to less stringent rules, a hat or fascinator will be compulsory for women. This marks a significant change to previous years, when female racegoers were simply advised that "many ladies wear hats". Strapless or sheer-strap tops and dresses will be banned. For men, a suit and tie will now be imperative. The less formal Silver Ring will not be affected by the changes.
Charles Barnett, Ascot's chief executive, said the overarching intention was to be "as helpful as possible" to visitors and to assist racegoers in understanding what is "cherished" about the dress code at Royal Ascot. He said: "It isn't a question of elitism and not being modern in a world where there is less and less requirement to dress smartly - far from it. We want to see modern and stylish dress at Royal Ascot, just within the parameters of formal wear, and the feedback we have received from our customers overwhelmingly supports that."