Prince William, Prince George and Princess Charlotte recently attended one of Taylor Swift's Eras Tour shows in London.
After Prince William brought his and Kate Middleton's two eldest children to Taylor Swift's Eras Tour concert in London on Friday, royal expert Katie Nicholl is giving ET rare insight into the Prince of Wales' friendship with the "Karma" crooner.
Talking with ET on Wednesday, Nicholl shares that while a source recently said Princess Charlotte is the biggest Swiftie in the family, she may have competition for that title with her father -- who has a history with the 14-time GRAMMY Award winner.
"I'm not sure that Princess Charlotte is the biggest Swiftie of the family or whether it's actually Prince William. I know he loves her music," Nicholl says, adding of 34-year-old Swift and 42-year-old William, "They have a great connection, they're good friends."
So if Princess Charlotte really is looking to give her father a run for his money, she has several years of fandom, one hilarious video of herself "shaking it off" and an on-stage experience with the hitmaker to make up.
In 2013, the Prince of Wales and Swift sang alongside Jon Bon Jovi at the Centrepoint Gala Dinner at Kensington Palace in London, belting out "Livin' on a Prayer." Photos and videos from the night show the trio having a blast together while singing Bon Jovi's classic tune, and embracing after receiving a standing ovation from the crowd.
Nicholl says she was in attendance that night and that Swift and William, it was just the start of what would be a long and prosperous friendship.
"They remained good friends [after that night]. So good, in fact, that William got the royal treatment [at the Eras Tour]. He got the VIP tickets and he had -- he had a wonderful time backstage with Taylor, where she took a selfie of her with the children and Prince William."
Prince William and Princess Kate, 42, share three children: George, 10, Charlotte, 9, and Louis, 6. Kate -- who has been away from royal duties for several months after a shock cancer diagnosis -- stayed at home with the youngest member of their brood.
Nicholl adds, "There is clearly a lovely connection between these two. But William is not just a fan of Taylor, he's a huge fan of her music, too."
The feeling is clearly reciprocated as Swift -- who has mostly used her Instagram page these days to post professional photos from the tour -- devoted a whole Instagram grid post to wish Prince William a happy 42nd birthday while sharing the selfie they took backstage. The picture also served as her first post with boyfriend Travis Kelce, who she began dating last summer.
"Happy Bday M8! London shows are off to a splendid start," Swift wrote in the caption of the post, which has since received nearly 10 million likes.
For Travis, 34, who had never met the Prince of Wales until then, the experience was equally exciting. He addressed the introduction to Prince William in the latest episode of his podcast, New Heights, which he co-hosts alongside his brother, Jason Kelce.
"Dude, he was the coolest motherf**ker," Travis told 36-year-old Jason, who was also at the Eras Tour show at Wembley Stadium on Friday and was joined by his wife of six years, Kylie Kelce.
Travis added of the future king of England, "He was so cool."
Jason -- a former Philadelphia Eagles player who adorably spotted a fan wearing his jersey outside of Wembley -- also got to meet the royal family and added his own two cents on the interaction.
The elder of the two Kelce brothers said he assumed it would be "hard to be a down-to-earth human being" as a member of the royal family, but that they "came off that way completely."
As for how the interaction between the actual royal family members and "Miss Americana and the Heartbreak Prince" came together, a source told ET on Monday that Princess Charlotte is really responsible for getting the gang together to celebrate William's birthday at the concert.
"Princess Charlotte is the huge Taylor Swift fan in the family. She has wanted to go to the concert for months, but with her mum being treated for cancer, this was a really special day to be able to celebrate her Papa's birthday like this," the source shared.
"Charlotte absolutely loved [the concert]," the source added. "They are really wonderful children. William and Kate have done a fantastic job raising all three of them. They understand and appreciate how lucky they are meeting someone like Taylor Swift."
Over on William and Kate's official social media accounts, the family expressed their gratitude for what is sure to become a cherished memory. "Thank you @taylorswift for a great evening!," the posts read. "#LondonTSTheErastour."
While it was Charlotte who appeared to be driving the family's plans to attend the Eras Tour, Nicholl tells ET she is calling it a draw on whether it's the Prince of Wales or his young daughter who takes the title of "Chief Swiftie" inside Kensington Palace.
"I think there's probably a bit of competition between William and Charlotte as to who's the biggest Swiftie," Nicholl quips.
For more on Prince William's outing with his kids at the Eras Tour, check out the links below.
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