France has declared a significant rise in coronavirus cases - as researchers have said traces of COVID-19 are once again being found in Paris's sewage system. Photo: Internet Source

The country's health ministry reported 1,062 cases on Thursday - nearly double the 584 seen on Tuesday.

It marked the second day in a row that the country had seen a sizeable rise compared to earlier in the week, after a series of localised flare-ups prompted officials to make mask-wearing compulsory in enclosed public spaces.

The health ministry said there had been a 66% increase in cases over three weeks, and that a lag between people becoming infected and displaying symptoms meant the virus had likely been circulating "for several weeks already". 

The mayor of popular beach resort La Baule-Escoublac has warned the tens of thousands of tourists flocking there to observe social distancing, after the town was flooded with holidaymakers.

Meanwhile, researchers say some locations in Paris that had been testing negative for traces of coronavirus in their wastewater have been testing positive again since the end of June.

The virus traces had initially been detected at the beginning of the outbreak, but vanished when France went into lockdown back in March.

Early studies by scientists across the globe have suggested that sampling sewage for signs of COVID-19 could help estimate the number of infections in a geographic area without having to test every person, and could act as an early indicator before people are even diagnosed or sent to hospital.

Parisian researchers said in April that sampling wastewater in the French capital for a month reflected the rising and falling number of infections there.

Laurent Moulin, who heads a research laboratory run by public water utility company Eau de Paris, insisted the new findings on their own did not mean the virus is spreading again.