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Spanish virologists believe they may have found traces of COVID-19 in a sample of waste water collected in Barcelona in March of last year.
If that is proven to be the case, it would mean the discovery was made nine months before the coronavirus was first identified in China.
It is a single sample, however, and more would be needed to confirm the result, said Dr Joan Ramon Villalbi of the Spanish Society for Public Health and Sanitary Administration.
He added: "When it's just one result, you always want more data, more studies, more samples to confirm it and rule out a laboratory error or a methodological problem."
Despite there being the potential for a false positive, because of the virus's similarities with other respiratory infections, Dr Villalbi said the finding was "definitely interesting, it's suggestive".
The research has been submitted for peer review after the sample was found by a team from the University of Barcelona, which has been testing waste water since mid-April.
"The levels of SARS-CoV-2 were low but were positive," research leader Albert Bosch was quoted by the university as saying.
Previously, their earliest discovery of the virus was in Barcelona on 15 January - 41 days before the first case was officially reported.
Professor Bosch said early patients were probably wrongly diagnosed with common flu, and that earlier detection of COVID-19 would have improved the response to the pandemic.
Sewage samples taken in Italy suggest the virus may have been there as early as December .
It is thought COVID-19 may have been present in France in the same month.
Spain has been one of the countries hardest-hit by the virus, with more than 28,000 deaths, and had one of the world's strictest lockdowns as residents were kept inside apart from trips for food, medicines and essential jobs.
But the country has begun to reopen and even welcome tourists , with Britons among those able to visit without quarantining despite the UK still reporting hundreds of deaths each week .