Peter Phillips and his girlfriend, Lindsay Steven, joined Peter's sister, Zara Tindall and her husband, Mike, at the opening day of Royal Ascot.
The couple were spotted in the parade ring as King Charles, Queen Camilla and Peter's mother, the Princess Royal, arrived in the carriage procession.
Peter, 45, who is now head of partner acquisition for the sports and entertainment rights agency CSM Sport & Entertainment, looked smart in a morning suit and top hat, while Lindsay, 42, opted for a silk blue tea dress and a matching hat.
Meanwhile, equestrian Zara, 42, looked lovely in a botanical print dress by Australian brand, Leo Lin, and a boater-style hat.
Peter and Lindsay became close when both their marriages broke down and have been dating for nearly two years.
A friend of the King’s nephew told HELLO!: "He is in a great place at the moment and could not be happier in either his private or personal life. Peter and Lindsay just look so natural and great together and are incredibly happy. They are both very private people, but the pictures of them at the Coronation Concert and those smiles just say it all.
"Lindsay really is a breath of fresh air and it’s clear to see both Lindsay and Peter are loving life. On top of that, he has returned to the exciting world of F1," added the friend. "It is a boy's dream to be working in that environment.
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"Peter has always been extremely close to Charles, both when he was the Prince of Wales and now the King. It means a lot to Peter that Lindsay was allowed to take her place in the royal box by his side for a very important part of the coronation weekend."
The couple have joined the royal family for several high-profile events over the past year, including the late Queen Elizabeth II's Platinum Jubilee celebrations last June, as well as the Epsom Derby, Royal Ascot and Wimbledon.
Peter was previously married to Autumn Kelly from 2008 until their separation in 2019. The pair, who share daughters, Savannah, 12, and Isla, 11, finalised their divorce in 2021.
On the opening day of the Berkshire meet, Princess Anne presented the winners' trophy in the St James's Palace Stakes.
The King and Queen were also joined by members of Camilla's family, including her son Tom Parker Bowles, her sister Annabel Elliot, and her nephew Sir Ben Elliot and his wife Mary-Clare.
The Duke and Duchess of Gloucester, and Princess Beatrice and Edoardo Mapelli Mozzi, also stepped out for day one of the races.