Prince Michael and Princess Michael's daughter, 43, looked elegant in floral shirt dress and a wide-brimmed peach hat as she travelled in the carriage procession at the start of the Berkshire races.
It comes after Lady Gabriella made a low-key appearance at Trooping the Colour last Saturday, where she was pictured watching the flypast from inside Buckingham Palace, as the senior royals gathered on the balcony.
It has been a difficult year for the British writer following the unexpected passing of her husband and financier, Thomas Kingston, who was found dead at the age of 45 at his parents' home in the Cotswolds on 25 February.
The King and Queen are said to have been quietly supporting Lady Gabriella and she has also reportedly moved back into her parents' home at Kensington Palace.
Also joining the royals on the first day of Royal Ascot were Camilla's sister, Annabel Elliot and her nephew, Ben Elliot.
Lady Gabriella was seated in the second carriage with Princess Anne and her son, Peter Phillips, as well as John Warren - the King and Queen Elizabeth II's racing manager.
The King sported a grey morning suit with a sunny lemon waistcoat, while the Queen looked beautiful in a cobalt blue ensemble with a matching hat and a sapphire brooch.
The King and Queen wished racegoers the "best of luck" in chasing the "thrill of a Royal Ascot winner" as the famous meet began.
Writing jointly in the official programme, Charles and Camilla said: "It gave us immense pleasure and joy to witness Desert Hero's victory in the King George V Stakes last year, particularly as he was bred by Her late Majesty.
"We wish all the participants this week the very best of luck in seeking that unique thrill of a Royal Ascot winner.
"We also hope that those here at the racecourse and watching from home will enjoy five days of the finest racing that this sport has to offer."
A full-length look at the King and Queen's race day attire as they stepped out of the carriage.
It was founded by Queen Anne in 1711 after she declared Ascot ideal for "horses to gallop at full stretch" and has today become a major social event as well as a sporting attraction.
Peter Phillips is a regular at Royal Ascot, and has recently confirmed his new romance with NHS nurse Harriet Sperling.
The pair were spotted packing on the PDA at the polo earlier this month, as they were joined by Harriet's daughter and Peter's daughters, Savannah and Isla.
Zara always wows us with her race days looks and the first day of Royal Ascot was no exception! The equestrian looked stunning in a puff-sleeve yellow pebble print shirt dress by Laura Green.
She wore her blonde bob in an intricate updo with a boater-style hat by Sarah Cant and accessorised with her Sauvereign Aphrodite Clutch.
Meanwhile, former rugby star Mike looked dapper in a morning suit with a pastel tie to match his wife's ensemble.
It was a right royal turnout on Tuesday as Princess Eugenie and the Duchess of Gloucester joined their family members.
While Birgitte opted for an all-navy ensemble, Eugenie looked pretty in a sleeveless mint number with a boater-style hat, a floral hair corsage and beige Aquazzura heels.
Queen Camilla beamed as she took to the stage to present The St James’s Palace Stakes to Jockey Sean Levey, rider of Rosallion.
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