Vatican law infringes on free speech, said the Italian investigative journalist accused of leaking confidential documents.

Emiliano Fittipaldi called the law, which only applies within the Vatican City state, "medieval" after his new book Avarice is under investigation for publishing leaked documents from the Holy See. Fellow Italian journalist Gianluigi Nuzzi is also under investigation for his book, Merchants in the Temple.

"From my point of view they are crazy charges, in the sense that in no democratic state, in no Western democracy, are there such restrictive laws on press freedom and expression," said Fittipaldi.

The two are accused of crimes against the Vatican state in laws brought in by Pope Francis in 2013 which intended to prevent documents being leaked. It punishment can be up to eight years in prison.

In addition to investigating the two journalists, two people involved in advising the pope on reforms were arrested on suspicion of leaking the documents. Spanish priest Lucio Angel Vallejo Balda and laywoman Francesca Chaouqui were part of a temporary advisory commission and were arrested earlier this month.

However Fittipaldi remained defiant that his decision to publish was the right one.

"[The books] narrate the financial riches, the economic scandals within the Vatican," he said. "It's news, I think, that is fundamental to recount. It's important for the faithful and the nonfaithful around the world."

It is the first time Francis' 2013 law will be put to the test. Fittipaldi suggests he was a scapegoat in a bid to deter other leaks.

"They want to create an internal precedent, as a way to stop other leaks in the future. They don't care at all about what we think of these medieval rules."

The episode has been referred to as 'Vatileaks II' after another scandal in 2012 when Nuzzi published documents leaked by the then Pope Benedict XVI's butler.