Formal dress codes are being dropped with immediate effect at all 15 racecourses owned by the Jockey Club, including Aintree and Cheltenham.
The move is part of a drive to make horse racing more "accessible and inclusive".
Spectators will be encouraged to dress as they feel "most comfortable and confident".
Replica sports shirts or "offensive" clothing of any kind will still not be allowed.
The changes will apply to a total of 342 fixtures, including the Grand National meeting.
The Queen Elizabeth II Stand at Epsom on Derby Day will continue to require either morning dress, including top hats for men, or formal daywear on Derby Day.
Royal Ascot is not covered by the Jockey Club, and there have been no changes announced to its traditional dress code.
The debate over dress codes on British racecourses resurfaced last year when two racegoers were initially refused entry to Sandown's most expensive enclosure in April because they were wearing trainers.
Jockey Club chief executive Nevin Truesdale said: "It's really important to us to be accessible and inclusive.
"We hope that by no longer placing an expectation upon people of what they should and shouldn't wear we can help highlight that racing really is for everyone.
"When we reviewed this area of the raceday experience, it has been clear to us that enforcing a dress code seems rather outdated in the 21st Century in the eyes of many of our racegoers.
"Of course that doesn't mean we are discouraging people from dressing up for a day at the races if they want to.
"This is about giving people a choice and the opportunity to come racing dressed however they feel most comfortable and confident, while also bearing in mind the challenges regularly presented by the British weather."
Truesdale said there have already been a significant number of fixtures where dress codes have not been in place.
He said: "It is a common misconception that a day at the races has always required you to dress in a certain way, regardless of the fixture.
"In fact, even at really high-profile days like the Cheltenham Festival, that has simply not been the case and our only recommendation has been to dress appropriately for the weather."
Jockey Club racecourses: Aintree, Carlisle, Cheltenham, Epsom, Exeter, Haydock, Huntingdon, Kempton, Market Rasen, Newmarket (Rowley Mile & July Course), Nottingham, Sandown, Warwick, Wincanton.
