Pope Benedict XVI resigned after an internal investigation informed him about a web of blackmail, corruption and gay sex in the Vatican, Italian media reports say.
Three cardinals were asked by Benedict to verify allegations of financial impropriety, cronyism and corruption exposed in the so-called VatiLeaks affair.
On December 17, 2012, they handed the pontiff two red-leather bound volumes, almost 300 pages long, containing "an exact map of the mischief and the bad fish" inside the Holy See, La Repubblica said.
"It was on that day, with those papers on his desk, that Benedict XVI took the decision he had mulled over for so long," said the centre-left newspaper. It said its article was the first of a series.
Panorama, a conservative weekly, did not speculate about the motives behind Benedict's resignation, but its story about the contents of the confidential report was broadly similar.
Vatican spokesman Father Federico Lombardi refused to "run after fantasies and opinions" and warned reporters: "Don't expect comments or rebuttals of what is being said on this issue."
La Repubblica quoted a man described as "very close" to the authors as saying the information it contained was "all about the breach of the sixth and seven commandments" - which say "thou shalt not commit adultery" and "thou shalt not steal".
The cardinals were said to have uncovered an underground gay network, whose members organise sexual meetings in several venues in Rome and Vatican City, leaving them prone to blackmail.
The secret report also delves into suspect dealings at the Institute for Religious Works (IOR), the Vatican's bank, where a new chairman was appointed last week after a nine-month vacancy, La Repubblica said, without going into details.
The newspaper said Benedict would personally hand the confidential files to his successor, with the hope he will be "strong, young and holy" enough to take the necessary action.
The authors of the secret report will not take part in the conclave because they are over 80 years old, past the age limit for the meeting. But Panorama said they were likely to inform other cardinals about what they have uncovered.
Their findings "will condition the conclave" as it will have to elect "a pope immune to blackmail, so that he can start the clean-up operation that (Benedict) entrusted to his successor".