Princess Anne broke royal protocol towards Queen Camilla during Monday's Commonwealth Day Service.
Queen Camilla greeted the senior royals inside Westminster Abbey when they arrived for the service.
The Duke and Duchess of Edinburgh both met the Queen with a kiss on each cheek, followed by a bow and a curtsy.
However, the Princess Royal did not curtsy to her sister-in-law, choosing only to kiss Camilla on her cheeks before walking away.
Princess Anne and Duchess of Edinburgh joined Queen Camilla for the royal service
Reuters
Royal protocol dictates that anyone with an HRH title should bow to a sovereign and their spouse, which would include King Charles and Queen Camilla.
However, Princess Anne did not follow this rule during Monday's Commonwealth Day Service.
The Royal Family seem to have taken a more relaxed approach under the reign of King Charles III.
Queen Camilla turned around and began talking to someone else after the slightly awkward interaction.
Queen Camilla wore a Bruce Oldfield blue coat dress during the ceremony
ReutersThe Queen, 76, led the Royal Family at Westminster Abbey whilst wearing a Bruce Oldfield blue coat dress.
She paired this with a Philip Treacy hat, Russel & Bromley boots, a Bottega Veneta cassette bag, and an aquamarine brooch that belonged to Queen Elizabeth II.
Sophie, Duchess of Edinburgh looked beautiful for the ceremony, wearing a Suzannah London coat and matching dress.
She paired these with a Jane Taylor hat, a Sophie Habsburg clutch bag and Stuart Weitzman shoes.
Queen Camilla, Prince William, Prince Edward, Duchess of Edinburgh and Princess Anne at royal service
PA
Meanwhile, Princess Anne stunned in red during the Commonwealth Day Service.
She wore her Shibumi Grace coat and scarf which she has worn several times before.
Prince William also joined his stepmother at the Commonwealth Day Service on Monday.
He attended the ceremony without his wife, Princess Kate, who had a "private appointment" according to The Mail.
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The members of the Royal Family entering Westminster Abbey
Reuters
The Princess of Wales temporarily stepped back from royal duties two months ago following abdominal surgery.
Kate, 42, is not due to resume duties until after Easter.
She is currently continuing her recuperation at Adelaide Cottage in Windsor with her husband Prince William and their three children.