The couple’s romance had been a secret up until that point—and William was reportedly furious.

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As the Prince and Princess of Wales, Prince William and Kate Middleton have one of the most publicized relationships. Although the couple constantly makes front-facing appearances—a pivotal aspect of their royal duties—their partnership hasn’t always been out in the open. When the two of them first started dating while enrolled at St. Andrews, their relationship was “shrouded in secrecy,” according to Robert Jobson’s new book, Catherine, The Princess of Wales: The Biography. Even though members of the student body suspected that Kate and William were dating, a drinking game and one of William’s ex-girlfriends were responsible for forcing them to publicly confirm their romance.

Jobson writes that while William and Kate were at a dinner party with friends, the group decided to play “Never Have I Ever,” a drinking game in which players must admit experiences they haven’t done before. An ex-girlfriend of William’s, Carly Massy-Birch, was one of the guests of the soirée, and when it was her turn, her confession seemingly targeted William and Kate. “Perhaps unwittingly, Carly exposed William’s secret love affair with Catherine, much to the prince’s chagrin,” Jobson says in his book.

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Another royal novel, The Making of a Royal Romance by Katie Nicholl, provided more insight into what Massy-Birch said. “She announced, ‘I’ve never dated two people in this room,’ knowing full well that William was the only one who had because Kate was sitting next to him,” one of the dinner party guests told the author. The group was supposedly “in shock” at her bold move—especially William. The guest recalled that Willam “shot a thunderous look” at his ex-girlfriend and whispered, “I can’t believe you just said that.” “We knew they were together, but it was the first time William confirmed his and Kate’s relationship in public,” the insider revealed.

Up until that point, William and Kate were dating in secret. However, since the two of them spent a lot of time together, like walking to class and shopping at the grocery story, per Jobson, much of the student body had a feeling they were an item. When they first met during their first year at the university in 2001, their relationship was platonic. According to William’s engagement interview in 2010, it wasn’t until they starting living together in college that their connection turned romantic. “We moved in together as friends,” William explained. “We lived with a couple of others, as well, and it just sort of blossomed from there, really. We just saw more of each other, hung out a bit, and did stuff.”