The publication occurred on Italy's tax service website and was done by Romano Prodi's outgoing government just days before leaving office. The information was available for 24 hours but was taken down after outraged criticism and an official complaint from Italy's privacy watchdog.
ADOC, the Italian consumer protection agency, said that the release of the information was not only illegal but put citizens in danger.
"It's a clear violation of privacy law," it told news agency Reuters. "There is a danger for an increase in crime and violence as the data are an irresistible source for criminals."
The UK Government has been heavily criticised in recent months because of a series of data security lapses involving the loss of data belonging to 25 million child welfare payment recipients as well as members of the armed forces and applicants for driving licences.
All the incidents were accidental, though, and none compares to the Italian government's decision to publish such a massive amount of such sensitive information.
The then-government was unrepentant in the aftermath of the publication of the information 10 days ago. "It's all about transparency and democracy. I don't see the problem," deputy economic minister Vincenzo Visco told Italian newspaper Corriere della Sera.
Visco said that the move was in line with laws from the 1970s which allow people to look up tax records at municipal offices.
Italy's data protection commissioner Francesco Pizzetti agreed with ADOC that the fact that the data will probably always now be available was dangerous. "It's one thing to make data available in response to precise requests, another to publish it in this way," he told journalists.
Though the official site no longer carries the records, many people downloaded huge chunks of the data, which consisted of 38 million tax returns from 2005. Files claiming to be tax records are being traded on file-sharing services.
Prodi's government claimed that the move was designed to improve earnings transparency in a country which reportedly struggles with tax evasion.
A report from the govermnent last year said that the sum lost to tax evasion is equivalent to 7% of Italy's gross domestic product.