Harry Lopes, the husband of Queen Camilla's daughter Laura and chief executive of Eden Renewables, was spotted among the guests at Tuesday evening's White House State Dinner in Washington.
The appearance marked a notable departure from the typically low profile maintained by the Queen's relatives at formal occasions.
Approximately 120 attendees gathered for the event, which brought together an eclectic mix of business titans, sporting figures and political heavyweights.
The renewable energy executive's presence at such a high-profile diplomatic function raised eyebrows, given how seldom members of Camilla's side of the family participate in official engagements.
Queen Camilla's son-in-law invited to White House
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Mr Lopes married Laura Parker Bowles in 2006.
The father-of-three previously worked as an accountant before taking the helm at the renewable energy firm.
His lineage connects him to considerable wealth through the Astor dynasty.
Mr Lopes's mother, Sarah Violet Astor, was the granddaughter of the 2nd Baron Astor of Hever.
Harry and Laura Lopes pictured back in 2015
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The couple share three children together: Eliza, who arrived in 2008, followed by twin boys Gus and Louis the subsequent year.
It remains unclear what role brought Mr Lopes to the prestigious dinner.
The Queen's family members have largely avoided the spotlight at state functions, with their most recent public appearance being the May 2023 Coronation ceremony.
The guest list featured some of the most prominent names in technology.
Queen Camilla wore a Fiona Clare gown on Tuesday evening | GETTYLATEST ROYAL NEWS:
Queen Camilla, King Charles, Donald Trump and Melania Trump pictured at the State Dinner | REUTERSThis included Amazon founder Jeff Bezos, Apple chief Tim Cook, Salesforce boss Marc Benioff and Nvidia's Jensen Huang, all in attendance, alongside financier Steve Schwarzman.
Professional golfer Rory McIlroy was also present, as were numerous Republican senators, conservative Supreme Court justices and members of the Trump administration.
King Charles used his speech to inject humour into proceedings, joking about the 1814 burning of the White House by British forces.
He quipped: "I am sorry to say that we British, of course, made our own attempt at real estate redevelopment of the White House in 1814."