It is understood the mix-up happened at the Dumbarton plant of Chivas Brothers during the night shift on February 26 while equipment was being cleaned, the BBC reported.
"Instead of draining away waste water, the workers on duty somehow flushed out thousands of litres of bulk whisky," the report said.
Around 18,000 litres of Scotch poured into a local sewage works after the gaffe by staff at Chivas Brothers bottling plant, The Scottish Sun reported.
Chivas Brothers - which employs 600 workers at the plant and produces the world's second biggest-selling brand, Ballantine's - said it was investigating an accidental release of spirit.
More From This Section
A company statement said: "We are currently investigating an accidental loss on the 26th of February at our Dumbarton site, where some spirit was released to the local water treatment plant.
"There has been no release of spirit to the River Leven or any other local water course. We have informed Scottish Water and all other relevant authorities."
The apparent bungle happened during a routine cleaning operation in the early hours of Tuesday at the whisky plant.
He said bosses were "livid", adding: "The guys responsible will be lucky to keep their jobs.