Italian scientists say sewage water from two cities contained coronavirus traces in December, long before the country's first confirmed cases.
The National Institute of Health (ISS) said water from Milan and Turin showed genetic virus traces on 18 December.
It adds to evidence from other countries that the virus may have been circulating much earlier than thought.
Chinese officials confirmed the first cases at the end of December. Italy's first case was in mid-February.
In May French scientists said tests on samples showed a patient treated for suspected pneumonia near Paris on 27 December actually had the coronavirus.
Meanwhile in Spain a study found virus traces in waste water collected in mid-January in Barcelona, some 40 days before the first local case was discovered.