William was visiting Babcock Vehicle Engineering firm in Walsall, West Midlands when he revealed that the royals had spent the bank holiday weekend, and Charlotte's birthday on Sunday, at their country home in Norfolk.
The Duke, 38, was talking to Jenna Jackson, the company's lead HR business partner and a mother-of-two, who asked him if Charlotte had enjoyed her birthday.
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"She had a lovely day, thank you," said William, his eyes lighting up. "Last year it was her birthday in lockdown but this year we were able to have one other family over. They grow up very fast. It was great fun."
While the Cambridges were marking the day in private, William and his wife Kate followed tradition by sharing a new birthday portrait of their daughter with the public. In the photo, taken by the Duchess, Charlotte was pictured in a floral dress by one of Kate's go-to childrenswear designers, Rachel Riley.
The birthday girl, who is affectionately known as Lottie to her family, could be seen grinning as she stood outdoors in her pretty dress with her hair down.
Kate took this sweet photo of Princess Charlotte to mark her sixth birthday
The royals usually share official images on their Twitter and Instagram accounts, but last weekend William took part in a social media blackout in a stand against racism and abuse online.
Taking to Twitter, the future King explained: "As President of the FA I join the entire football community in the social media boycott this weekend. W." Charlotte's birthday portrait was therefore not shared on their platforms.
On Tuesday, William was visiting Babcock to recognise the work the company has done in keeping the police, military, air ambulances, other emergency services, civil service and other organisations running during the coronavirus pandemic. The company has also produced ventilators for the NHS during the pandemic and made air ambulances Covid-secure by fitting screens.
When asked by staff if he was a fan of his police bodyguards' cars, William joked about his entourage's driving skills, saying: "The guys really rate them. Some of their driving skills need to be questioned. Don't tell them I said that."
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