As the clerics were preparing to take their first vote in May inside the Vatican's Sistine Chapel, which was fitted with jamming equipment to prevent outside communications, security officials picked up an active mobile connection.
The cardinals stared at each other incredulously, then one of the older clerics discovered he had a phone in his pocket and handed it over, according to The Election of Pope Leo XIV, a new book by two long-time Vatican correspondents released on Sunday.
The book does not name the cardinal or suggest he had any motive for keeping his phone, saying the moment left him "disoriented and distressed".
The scene was "unimaginable even for a film and never before seen in the history of modern conclaves", authors Gerard O'Connell and Elisabetta Pique wrote.
One such film, the 2024 hit Conclave, imagined a tangled web of intrigues during the fictional selection of a pontiff.
The unprecedented 2025 discovery of a phone was in its own way more startling than anything portrayed in that movie, O'Connell told Reuters.
"Reality (was) better than fiction," he said.
Clerics taking part in a conclave take a vow not to communicate with the outside world and surrender their phones and all other communication devices for the duration of the proceedings, which can last for days.
The Vatican press office did not respond to a request for comment about the book, which offers behind-the-scenes details of one of the world's most secretive elections.
The cardinals met in a two-day conclave from May 7-8 under an intense global spotlight to elect a successor to Pope Francis, who died in April after 12 years leading the 1.4-billion-member church.
Two candidates immediately emerged as frontrunners inside the conclave, the book said.
One was Italian Cardinal Pietro Parolin, a long-time Vatican official. The other was Cardinal Robert Prevost, a figure mostly unknown outside church circles who would emerge as Pope Leo, the first pontiff from the US.
On the first vote in the conclave, held on May 7, Prevost received 20 to 30 votes, an unusually large number, the book said.
Cardinal Luis Antonio Tagle, from the Philippines, also seen as a favourite going into the election, received fewer than 10 votes in the conclave.
On the fourth ballot on May 8, Prevost won with 108 votes.
Tagle was sitting next to Prevost as the final vote was being tallied and offered the future Pope a cough drop to soothe his throat, the book said.
