Captain Mark Phillips, 77, stepped out on Saturday to witness his son Peter exchange vows with NHS nurse Harriet Sperling at All Saints' Church in Kemble, Cirencester.
Princess Anne's former husband was accompanied by Florence Standaert, the Belgian equestrian with whom he had been romantically linked for several years.
According to Tatler, the pair attended the ceremony together, though they were not photographed side by side.
Neither Captain Phillips nor Ms Standaert has ever publicly acknowledged being in a relationship.
Should the reports prove accurate, Peter Phillips's wedding would represent their most prominent joint appearance at a public occasion to date.
Captain Phillips and Ms Standaert share deep roots in the equestrian world, which appears to have brought them together.
The pair were first spotted at the Burghley Horse Trials in 2022, with sources at the time describing Ms Standaert as his girlfriend.
Their paths may have crossed two years earlier when Captain Phillips designed a cross-country course at Waregem in Belgium, an event Ms Standaert was running following the death of her former partner Thomas Ryckewaert in 2019.
Captain Phillips himself boasts an impressive equestrian pedigree, having claimed Olympic gold with the British team at the 1972 Munich Games.
Ms Standaert is similarly accomplished in the sport.
Princess Anne was also present to celebrate her son's nuptials, arriving with her husband, Vice Admiral Sir Timothy Laurence.
The Princess Royal, 75, opted for a striking yellow ensemble paired with a coordinating hat and ribbon.
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Anne and Captain Phillips married in 1973 and share two children, Peter and Zara Tindall.
The couple announced their separation in 1989, with Buckingham Palace stating at the time: "Her Royal Highness, the Princess Royal, and Capt. Mark Phillips have decided to separate on terms agreed between them.
"There are no plans for divorce proceedings."
Their divorce was finalised in 1992, though Captain Phillips remained at Anne's Gatcombe Park estate for another 21 years.
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