The outbreak came to light on Friday July 24th, when scores of people in the small Jura town reported suffering from diarrhoea and vomiting, leading the town authorities to test the water supply.
At a press conference on Monday at the town hall, authorities confirmed that the illness was caused by contaminated water containing E.Coli and other bacteria.
“It is now established without doubt that the cause of the gastro-intestinal epidemic which has affected the town of Le Locle since Friday comes from a contamination of the water supply by a norovirus,” authorities said in a statement.
“However we have not yet been able to discover where it came from.”
The number of people affected is put at 1,000, one tenth of the area's 10,000 population.
At least 80 residents have been treated by the emergency ward at Neuchâtel hospital, with one person admitted.
On Sunday the town's water reservoirs were treated with chlorine in an attempt to eradicate the virus, but it remains too early to say if the treatment has been successful.
In the meantime authorities are advising people to refrain from drinking tap water, or to boil it to at least 90 degrees beforehand, until further notice.
Residents should also avoid ingesting water during a shower, and avoid contact with the town's fountains.
Located in Switzerland's watchmaking valley, the UNESCO-protected town of Le Locle is a historically important part of the country's watchmaking history, which draws tourists from around the world.