Coronavirus may have been present in Italy as early as December, according to the country's National Institute for Health (ISS).
The virus was found in samples of sewage water in Milan and Turin, the institute said in a new report.
ISS researchers analysed 40 samples of wastewater collected from October 2019 to February 2020, and compared them to 24 control samples from September 2018 through June 2019.
The virus that causes COVID-19 was present in samples from Milan and Turin on December 18 and in Bologna on January 29. The samples were analysed in two different laboratories with two separate methods, the institute said.
Samples in January and February 2020 also tested positive, whereas samples from October and November 2019 tested negative.
A couple from Rome who had travelled from China became Italy's first recorded case of coronavirus in late January. The first recorded case in northern Italy was a man in Codogno, who had dined with a colleague from Shanghai.
"This research may help to understand the beginning of the virus's circulation in Italy," the ISS said in a statement.
The Italian report follows similar findings in other European countries. Researchers in Barcelona found coronavirus in wastewater dating back to 41 days prior to their first reported case.
Doctors at a hospital in France retested samples from patients who were hospitalised with pneumonia in December 2019, finding at least one man who tested positive. Researchers encouraged other hospitals to do the same.
Luca Lucentini, who leads the Italian health institute's department for Quality of Water and Health, said the Italian discovery does not mean "the main transmission chains that led to the development of the epidemic in [Italy] originated from these first cases."
"But a surveillance network in the area may prove to be valuable to controlling the epidemic," Lucentini added.
The institute has presented a proposal to the Italian Ministry of Health to surveil wastewater systems for coronavirus and hopes to launch a pilot study in tourist locations.