Millions of euros donated to the poor by Catholics around the world has been used to plug holes in the Vatican's finances, it has been claimed.

For every ten euros given every year under the centuries-old Peter's Pence donation scheme, six allegedly went straight into the Vatican's coffins before Pope Francis ascended to the papacy.

This is revealed in two explosive books about the Catholic Church which have been published this week, and have already been condemned by the Vatican.

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For every ten euros given every year by the faithful around the world under the Peter's Pence donation scheme, six allegedly went straight into the Vatican's coffins before Pope Francis' ascension, new books claim

The books depict a Vatican plagued by mismanagement, greed, cronyism and corruption and where Pope Francis still faces stiff resistance from the old guard to his reform agenda.

Gianluigi Nuzzi, one of Italy's best-known investigative reporters, has written 'Merchants in the Temple', which claims that the Catholic Church is guilty of diverting millions intended for the poor to plug administrative deficits and to fund the lavish lifestyle of some cardinals.

The management of Peter's Pence, a collection taken up yearly around the world and sent to Rome meant to aid the less fortunate, 'is an enigma cloaked in the most impenetrable secrecy,' he writes.

He claims that maintenance and restoration contracts were handed out at inflated prices, that Vatican real estate is worth seven times what it is listed on the account books and that the city-state's pension fund is fast approaching collapse.

'[For every ten Euro] two went to a reserve account already in credit to the tune of 400 million euros and only two to the pope for the good works it is meant to finance.'

Saviour: The books have been condemned by the Vatican, who says the claims in the book predate's Pope Francis and have since been dealt with

Another book published this week by fellow journalist Emiliano Fittipaldi makes a similar claim about the Peter's Pence scheme and says it raised 378 million euros in 2013.

A Vatican spokesman on Wednesday dismissed the content of the two new books as 'out of date and biased,' claiming that Francis's reforms had already addressed many of the issues raised

One highlight of Nuzzi's book, which was made available to Reuters before publication, is the transcript of a recording of the pope at a meeting in July 2013 - four months after his election - in which he complains to top Vatican officials about its murky finances.

'We have to better clarify the finances of the Holy See and make them more transparent,' he is quoted as saying in the recording, which the author says was made secretly by someone in the room.

'C-l-a-r-i-t-y. That is what's done in the most humble companies and we have to do it, too,' he says, adding that 'It is no exaggeration to say most of our costs are out of control'.

Nuzzi's new book has evoked painful memories of the 2012 'Vatileaks' scandal centred on documents fed to the media by then-pope Benedict XVI's butler.

Controversial claims: Gianluigi Nuzzi's book 'Merchants in the Temple' claims that the Catholic Church is guilty of diverting millions intended for the poor to plug administrative deficits and fund the lifestyle of cardinals

Nuzzi also played a central role in breaking that story, which contributed to Benedict's shock decision to retire the following year.

Vatican authorities this weekend arrested an Italian PR expert and a Spanish priest on suspicion of stealing and leaking classified documents to the media.

Church officials have not explicitly accused either Francesca Chaouqui or Lucio Angel Vallejo Balda of being behind the papal wiretap but have appeared to link the pair to the content of Nuzzi and Fittipaldi's books.

Chaouqui claimed in an interview on Wednesday that it was the priest who had made the illicit recording. She is thought to be cooperating with the Vatican investigation and has been released from detention, while Vallejo Balda remains in prison.

Francis's allies insisted that the leaks scandal would not affect his determination to push on with his reform agenda.

But Italian archbishop Nunzio Galantino acknowledged the pope must be feeling betrayed.

'I put myself in his shoes. No child of the Church, faced with such attacks, could remain indifferent,' Galantino said. 'Some people are clearly afraid of the renewal process the pope is carrying out.'