Royal Ascot gets under starter's orders on Tuesday amid heightened security measures.
Armed police will be patrolling the racecourse, and more extensive dog, bag and physical searches will be carried out this year in the wake of the terror attacks in London and Manchester.
Racegoers were warned that access to the course will take longer and to allow extra time.
Up to 300,000 people are due to descend on the Berkshire event over the next five days, including the Queen who has attended every year since she came to the throne in 1952.
Organisers advised punters via the Ascot website to expect tightened security, saying: "You will notice a stronger armed and uniformed police presence throughout the site and the vicinity."
They urged people not to bring unnecessary bags or luggage, adding: "We also ask for your vigilance on matters of security. If you see or hear anything suspicious, please bring it immediately to the attention of the police or our stewards."
Weather forecasters predicted scorching temperatures throughout the day, with highs of around 28C (82F).
The social occasion is as much an event for fashionistas as for racing fans, with an array of outlandish hats on display - particularly on Ladies Day on Thursday.
The colour of the Queen's hat is the subject of hundreds of bets - with bookmakers Ladbrokes citing blue as the favourite for the first day at 3 to 1.
For the first time, jumpsuits are being permitted as suitable attire for women in the Royal Enclosure - but they must be full length to the ankle.
The strict dress code means strapless, off the shoulder, halter-neck, spaghetti straps and dresses with a strap of less than one inch (2.5cm) are banned in the exclusive area.
Midriffs must be covered and fascinators are also forbidden.
Men must wear black or grey morning dress with a waistcoat and tie - but not a cravat - and a black or grey top hat.
This year, Royal Ascot has its first new enclosure for 100 years - the Village.
Described as "a pop-up summer scene", it has a bandstand and two other stages for jazz and funk in the day and bands and DJs in the evening, plus a village green.
At Royal Ascot last year, racegoers got through: 51,000 bottles of champagne; 160,000 glasses of Pimms; 131,000 pints of beer; 5,000 kilos of salmon; 6,000 Cornish crabs; 4,900 lobsters; 89,000 bottles of mineral water and 42,000 bottles of wine.
On Wednesday, the 91-year-old Queen will dash from the rescheduled State Opening of Parliament, after delivering the Queen's Speech, to the race meet - one of her favourite annual occasions - possibly via Windsor to meet lunch guests.
The monarch's address in the House of Lords is due to begin at 11.30am and the Queen is expected to make it in time for the first race at 2.30pm and may even be there for her traditional arrival ceremony, the carriage procession, at 2pm.
The State Opening was due to take place on June 19, but was delayed for two days after the inconclusive outcome of the General Election.
In 2001, the Queen opened Parliament on the same day as Ascot - and swapped the Imperial State Crown for a hat and changed out of her ceremonial gown at Buckingham Palace, before arriving by car 15 minutes before the first race.