Their friendship-turned-romance had been “shrouded in secrecy” before an underhanded move from William’s ex-girlfriend changed that at a college dinner party.

Though they’ve been together for over half of their lives now, when Kate Middleton and Prince William met as first-year students at the University of St. Andrews in the fall of 2001, they didn’t immediately begin dating.

Attached to other people at the time, the two forged a friendship first before eventually becoming a couple. Even after they were dating, it took them time to confirm it, royal biographer Robert Jobson writes in his new book, Catherine, the Princess of Wales.

“Among the student community, it was an open secret that they were a couple,” Jobson writes in the new book, out Aug. 6. He added that the pair often walked to class, browsed the local grocery store or spent nights at home listening to music together.

In the couple’s engagement interview in 2010, Prince William reflected on their time living together as roommates with a few other friends, saying, “We moved in together as friends. 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 more and did stuff.”

For a time, William and Kate’s relationship “remained shrouded in secrecy,” Jobson writes, until “a revealing moment occurred during a dinner party with friends attended by Carly Massy-Birch,” an ex-girlfriend of Prince William’s. The partygoers decided to play “Never Have I Ever,” a drinking game that encourages players to confess previous experiences.

“Perhaps unwittingly, Carly exposed William’s secret love affair with Catherine, much to the prince’s chagrin,” Jobson writes.

Royal biographer Katie Nicholl also detailed what went down that night in 2002 in her book The Making of a Royal Romance, and how, when it was Massy-Birch’s turn, “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,” a party guest told Nicholl.

The guest added that everyone was “in shock” by Massy-Birch’s move and that Prince William “shot a thunderous look” at her and whispered under his breath, “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 source said.

The tension didn’t last, and Massy-Birch was invited to Prince William and Princess Kate’s 2011 wedding, along with a handful of the royal’s other former girlfriends.