Pope Benedict XVI may have been hoping for some respite from the scandal which has engulfed his papacy, with a visit this weekend to Milan, where he celebrated an outdoor mass for a million faithful and took in a performance of Beethoven's ninth at La Scala opera house.

For the 85-year-old pontiff, the three-day trip outside the Vatican walls was a break from the Vatileaks scandal, which has seen his butler, Paolo Gabriele, arrested on suspicion of disclosing dozens of embarrassing letters alleging corruption and nepotism at the Holy See.

Gabriele is believed to be one of up to 20 whistleblowers trying to oust Benedict's powerful prime minister, secretary of state Cardinal Tarcisio Bertone, who has been accused of incompetence, covering up graft, and packing key Vatican posts with supporters.

The pope kept Bertone firmly at his side in Milan, sending a clear sign he is standing by his long-term collaborator, but the tension shows no sign of waning. On Sunday, La Repubblica published newly leaked Vatican correspondence with an anonymous covering note stating the whistleblowers still at large will not stop until Bertone – and the pope's personal secretary Georg Gänswein – are kicked out.

Gabriele, who was arrested in possession of crates of confidential letters addressed to the pope, has spent a week under guard in a "secure room" in the tiny city state and will be interrogated early this week. His lawyers are hinting he might name names.

"The Vatican is a hive of interests, different groups like Opus Dei and little transparency which heightens tension," said Gianluigi Nuzzi, the journalist who last month published the leaked letters in "Your Holiness", a book the Vatican describes as "criminal".

The stakes are high and time is short, since the winning side could have a huge say over who replaces the pontiff.

Letters published so far accuse Bertone of exiling a priest to a US post after he exposed graft at the Vatican and insinuate the cardinal was behind a gay whispering campaign against a newspaper editor.

But they also claim Bertone's biggest battles took place in Milan, involving high finance, a suicide, the Vatican's millions and the 77-year-old cardinal's alleged thirst for empire building outside the Holy See. "Bertone wanted to extend his authority beyond the Vatican and tighten his grip on Milan," Nuzzi told the Guardian.

In March 2011, Bertone abruptly fired Cardinal Dionigi Tettamanzi, the head of the Istituto Toniolo, a wealthy, religious foundation that controls the city's Cattolica university, an institution so influential three of its professors were appointed as minister in Mario Monti's technocrat government last November.

Bertone reportedly told Tettamanzi, a powerful former archbishop of Milan, he had Benedict's backing to sack him, but a furious Tettamanzi wrote directly to the pope demanding to know if Bertone had gone behind his back.

"This was a real clash of the titans," said Nuzzi of the battle, which ended with Tettamanzi resigning.

Next, Bertone allegedly ordered the head of the Vatican bank to bid for Milan's bankrupt San Raffaele hospital, founded by Don Luigi Verze, a confidant of former prime minister Silvio Berlusconi who enjoyed close ties to the Italian secret services. Wiretaps reveal he may have been linked to an alleged arson attack on a business rival.

By the time the Vatican bank chief, Ettore Gotti Tedeschi, checked the hospital's books, the senior accountant had killed himself, debts of €1.5bn were reported and a fraud inquiry was under way. Fearing what he would uncover, the Vatican banker refused to invest.

Lost in the news last week of the butler's arrest was the announcement that Gotti Tedeschi had been ousted by the board of the Vatican bank, a move seen by many as Bertone's revenge for his disobedience. Ironically, before he defied Bertone, Gotti Tedeschi had been among the cardinal's key appointments, one of many that reportedly rankled the Vatican's career staff of bureaucrats and diplomats.

Another criticism of Bertone is that he enjoyed too many foreign trips, yet while at home the Vatican stumbled from one gaffe to another. In 2007 the new archbishop of Warsaw withdrew an hour before his investiture when the Vatican realised he had spied for the communist police. After Benedict U-turned over bringing into the church a Holocaust-denying bishop, he was forced to admit a simple internet check would have revealed the man's credentials.

Italy's senior archbishop, Cardinal Angelo Bagnasco, stood at Bertone's side during this weekend's Milan papal visit, but in 2009 he was part of a group of senior churchmen who visited Benedict at his summer retreat reportedly to beg him to dump Bertone. The pontiff refused.

Two years later, the head of the Jesuits, Adolfo Nicolás, wrote to the pope enclosing a letter he had received complaining of "paralysing fear" inside the Vatican, where "money plays a key role."

"I don't think Bertone is a thief, he is just not up to the job," said one Vatican analyst, who decline to be namedspeaking on condition of anonymity.

Bertone's foes may have been further riled after a list of new cardinals appointed this year contained several allegedly pro-Bertone figures, increasing his influence in the college of cardinals when it votes for Benedict's successor.

But despite the growing dissent, it is unlikely Benedict will drop Bertone. Abandoning the man he entrusted in 2006 to run the Vatican bureaucracy while he focused on doctrinal issues would only weaken the pope's authority.

"I would like to renew my trust and my encouragement to my closest collaborators and all those who every day, with faith, a spirit of sacrifice and in silence help me to perform my ministry," Benedict said at his weekly public audience last week, a clear sign that for now at least, Bertone is staying put.

Meanwhile, Vatican investigators are debating whether to ask magistrates to haul in Nuzzi for interrogation after he declined to name his sources. "What crime have I committed?" said Nuzzi. "I am not interested in where the letters came from, just the news they contain."