Queen Elizabeth II said Royal Ascot would "remain one of Britain's finest sporting occasions" despite being forced behind closed doors as the meeting got under way on Tuesday.
The 94-year-old monarch is missing the five-day event for the first time in her 68-year reign.
The Queen, whose horse Estimate won the Ascot Gold Cup in 2013, will be watching on television from nearby Windsor Castle.
Despite no royal procession, the British monarch has maintained her tradition of writing the foreword to the racecard, sending her "best wishes to the thousands of racing professionals and enthusiasts who will join me in celebrating Royal Ascot this year".
"This year Ascot will feel different for many," she wrote. "It is so often a chance for friends, families and colleagues to gather together and enjoy a shared passion.
"I am sure however, that with the valiant efforts of the organisers, owners, trainers and stable staff it will remain one of Britain's finest sporting occasions and a highlight of the racing calendar."
All those permitted on course will have to undergo temperature checks and strictly observe social-distancing rules.
The first day boasts two Group One races, the Queen Anne Stakes and the King's Stand Stakes.
Frankie Dettori has a great chance to add to his 67 Royal Ascot wins in the Queen Anne on Terebellum, which he rides for his former employers Godolphin.
The warmest favourite of the day, though, is Battaash in the King's Stand Stakes.
He bids to win the five furlong (1,000 metres) sprint at the third time of asking, having finished runner-up twice.
Britain's Queen Elizabeth II is missing Royal Ascot for the first time in her long reign
A message of support for Britain's National Health Service at Royal Ascot