(Image: Aaron Chown / PA)
King Charles was reportedly "seismic with rage" after the Beckhams extended a birthday invite to Andrew Mountbatten-Windsor.
The claim is made in a new book about Sir David Beckham and Lady Victoria Beckham, and has been reported by the Express.
According to Alison Boshoff's Brand Beckham, the then Prince Charles was left furious after photographs emerged from daughter Harper Beckham's sixth birthday celebration at Buckingham Palace in July 2017.
Sir David and Lady Victoria, who live in Oxfordshire, attended the event after Harper was invited to a tea party hosted in Prince Andrew's private apartment at Buckingham Palace.
The invitation was reportedly arranged through Sarah Ferguson, Andrew's former wife.
Sir David later shared photographs from the occasion on Instagram, writing: "Happy Birthday to our special little young lady.
"Such a special girl who brings so much joy and happiness into our lives. Happy 6th birthday (I can't believe you're 6 already) have the most amazing day we love you pretty lady x".
The images showed Harper and friends outside and inside Buckingham Palace, with Princess Eugenie pictured alongside the children.
The book says the posts attracted criticism from former Queen Elizabeth II press secretary Dickie Arbiter, who said the Beckhams had "no right to be there" and asked: "Is it [Buckingham Palace] being opened up as a theme park?"
He added: "It devalues what the place is all about. Why do the Beckhams get special treatment?"
According to the book, a Palace insider described the social media posts as a "serious breach of protocol" and claimed: "Prince Charles is apparently seismic with rage and this will only deepen his feud with Prince Andrew."
The source also said: "Photos are absolutely not allowed in the Buckingham Palace quadrangle."
Another claim attributed to the insider was: "Prince William will not be amused."
The report states there was a perception that Ferguson had only "got away with" arranging the invitation because the late Queen was spending the summer in Scotland.
Sir David later addressed the controversy online, writing: "Just to be clear this wasn't the palace opening the gates for Harper's birthday party, this was a tea party where us and other guests were invited so it was a beautiful thing to do with my mum, Harper plus a few school friends."
He added: "We were honoured to be able to be there. Beautiful tea party."
Lady Victoria, who was away on a work commitment at the time, also shared a photograph of Harper dressed as Disney character Elsa and wrote: "Our little birthday princess x Kisses."
The reported row took place several years before Sir David was awarded a knighthood by King Charles.
In November 2025, the former England captain was knighted in recognition of his services to sport and charity, while Lady Victoria received the title that accompanies her husband's honour.
King Charles, who owns Highgrove House in the neighbouring Cotswolds, has since been seen alongside Sir David on a number of occasions as the football icon has taken on a greater role supporting royal charitable initiatives.