"The documents in my possession are proof of my honesty and my fight against the misdeeds of my lay superiors, covered up by certain cardinals," Nunzio Scarano said in a letter addressed to Pope Francis, cited by ANSA news agency.

The prelate, who was arrested on June 28th on suspicion of having acted as an intermediary for suspect transactions at the Vatican bank – otherwise known as the Institute for Religious Works (IOR) – proclaimed his innocence.

"I never laundered dirty money, I never stole, I tried to help someone who asked for help," he wrote from behind bars in Rome's Regina Coeli prison.

Scarano, who worked for years as an accountant for APSA, an agency that manages Vatican assets, said he had documents which would prove his good faith and asked the pontiff permission to present them to him in person.

Financial police accuse Scarano of acting as a front for suspicious payments made through the Vatican bank and "interrupting the traceability of money."

The investigators allege that Scarano had used Vatican bank accounts to make transfers on behalf of his friends, including an attempt to move €20 million on behalf of a Neapolitan shipowning family.

The IOR, which does not lend money, handles funds for all the Vatican departments, Catholic charities and congregations as well as priests and nuns around the world, and manages assets of some €7 billion.

According to the Vatican's Financial Intelligence Authority, there were six reports of suspicious financial activity in the Vatican in 2012.

Scarano holds several accounts at the bank. His lawyers insist that "Mr. 500" – as the cleric was nicknamed for the large amounts of €500 notes he was reported to carry on him – kept Church donations in the accounts.

Pope Francis has acted boldly to overhaul the Vatican bank's murky image, including installing a special five-member commission tasked with investigating the institute and reporting their findings directly back to him personally.

The pontiff also appointed cleric Battista Ricca to oversee the IOR's management – effectively placing one of his trusted allies in a key position.

However, the decision came back to haunt him last week when allegations emerged that Ricca had had gay relationships with male prostitutes during his time at the Vatican embassy in Montevideo in Uruguay and had kept a live-in lover.