The Princess of Wales challenged Wimbledon's strict etiquette during Sunday's men's singles final.
Princess Kate, who is patron of the All England Lawn Tennis and Croquet Club, attended the prestigious match alongside Prince William, Prince George and Princess Charlotte.
Sunday's match took place during sweltering temperatures and the harsh sun, which Kate sought to help maintain comfort by wearing a wide-brimmed hat from LK Bennett.
However, royal box guidelines explicitly advising against hats as "they tend to obscure the vision of those seated behind them."
Princess Kate wore a wide-brimmed hat from LK Bennett to Wimbledon on Sunday in apparent contravention of the rules
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The Princess, who became Wimbledon's patron in 2016, previously flouted the same rule in 2022 whilst wearing a yellow Roksanda dress.
The Kate Effect was seen in full following the Princess of Wales's appearance, as the £69 LK Bennett hat she wore sold out after being restocked last week.
Matching her hat, Kate also wore a bespoke royal blue V-neck midi dress by Roksanda featuring gathered fabric detailing on one shoulder.
She paired the elegant dress with tan Celia heels from Ralph Lauren, Cartier earrings and a gold Daniella Draper necklace.
Royal Box guidelines advise the hats 'tend to obscure the vision of those seated behind them'
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Princess Kate's decision to wear her hat was celebrated by Australian model Deborah Hutton, who founded the sun smart hat collection Canopy Bay in 2021, following her own experiences with skin cancer.
"Rules like not wearing hats are outdated and dangerous," she said, per The Sydney Morning Herald.
She added: "Melanoma rates are on the rise in the UK. At the tennis the sun is baking you. It's insanity not to wear a hat of some description."
Hutton praised Kate and Kidman as "two style leaders" who were "leading the way and showing that while tradition is important, some rules have to change."
Actress Nicole Kidman also ignored the rule as she sat with outgoing Vogue editor Anna Wintour
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"She is always going to look great but this is a choice that makes a difference," the model added.
However, she stressed that: "Her hat is not offering enough sun protection. You need fabrics that will protect you. Nicole has got it right."
Intriguingly, Kate did not wear a hat previous appearance at the Ladies final on Saturday to spectate Iga Swiatek and Amanda Anisimova compete.
For the occasion, she wore a bespoke cream dress from Self Portrait.
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Kate and Princess Charlotte also wore, almost, matching sunglasses to the event
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Adorning her jacket was a purple bow, which served as a thoughtful tribute to the All England Lawn Tennis and Croquet Club's traditional colours of green and purple, established in 1909.
During her Wimbledon appearances, the Princess of Wales was also seen wearing a pair of tortoiseshell cat-eye Ralph Lauren sunglasses to help cope with the sun.
Also wearing a similar accessory, but not a hat, was Princess Charlotte who sported a similar pair of shades with the classic cat-eye silhouette from Ray-Ban.
Charlotte had also been seen to break from conventions during her engagement at Wimbledon on Sunday as she sported bright pink nail polish.
While not a rule, members of the Royal Family normally opt for more neutral shades.