Royal Ascot has always been as much about the fashion as the horses but now racegoers will have to pass strict rules on attire before they can collect their winnings.
Organisers have introduced new rules and this year will be helped by a team of specially trained "dress code assistants".
Around 300,000 people will flock to Berkshire for the annual horse racing event which starts on Tuesday.
Fascinators, favoured by the Duichess of Cambridge, have been banned in the royal enclosure.
Instead hats must be worn or a headpiece which has a base of four inches (10cm) or more in diameter is also acceptable.
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, black or grey morning dress with a waistcoat and tie are now compulsory in the royal enclosure and cravats will not be allowed. A black or grey top hat and black shoes must also be worn.
In the grandstand, which is open to the public and subject to less stringent rules, a hat or fascinator will be compulsory for women.
The decision to tighten and clarify the dress code at the annual summer event comes amid criticism of sartorial standards which have been more loosely enforced in recent years.
A selection of waistcoats, ties, pashminas and other items will be available at the turnstiles for those who are not up to scratch.
The new dress code was first announced in January and organisers sent out reminders with badges and tickets.
Charles Barnett, chief executive of Ascot, said: "Ascot is Europe's flagship race meeting and one of the world's greatest sporting and social events. We have
tried to provide an appropriate dress code that is clear, workable and in line with the wishes of racegoers.
"Over the last few months, a huge amount of planning has gone in to making sure that the implementation of the new code goes as smoothly as possible.
"During the meeting itself, our team of special dress code assistants will help people with any queries that they may have."