“If the pontificate of Francis fails, it won’t be because of the attacks of conservative Catholics or the crisis in vocations or because of the declining number of faithful,” Nuzzi writes in the book. “It will be because of the financial collapse that is coming ever closer.”

The Administration of the Patrimony of the Apostolic See, a department of the Vatican which oversees real estate and investments, lost the equivalent of nearly $49 million last year — prompting officials to set up an emergency task force, according to Nuzzi.

He said the real estate portfolio was worth an estimated 2.7 billion euros ($3 billion) and was poorly managed. Some 800 properties were empty, while 15 percent of the 3,200 rented properties were being leased for free or for below-market prices.

“The deficit is recurring and structural and has reached worrying levels,” a Vatican official wrote in a memo obtained by Nuzzi. “We risk a default if no urgent steps are taken.”

Only a fifth of total donations go to the poor and the rest is held in bank accounts or used toward the debts of the Vatican’s governing body, the Curia, Nuzzi said.

Sex abuse scandals have rocked dioceses in the US, Ireland, Chile, Australia and other countries.

Pope Francis’ economic adviser slapped down allegations in Nuzzi’s book, calling them an attempt to discredit the pontiff.

“To say that the Vatican is at danger of default is false,” said Honduran Cardinal Oscar Maradiaga in an interview with Repubblica newspaper on Tuesday. “It seems to me there is a discrediting strategy underway.”

With Post wires