A watchdog has sent a report to the Procurator Fiscal after 6,000 local customers were left without water last summer.
On June 18, 2015, homes and businesses in Newarthill, Chapelhall, New Stevenston, Holytown and Dalziel Park reported concerns about the taste and smell of the water coming out of their taps.
They were advised not to cook, drink or wash with it for two days and nine schools were also closed as a precaution while Scottish Water investigated, revealing the supply had been contamination from an oily-based substance.
Hundreds of people were forced to queue for bottled water at collection points in Newarthill and New Stevenston, while deliveries were also made to households.
In her annual report, Sue Petch, Drinking Water Quality Regulator for Scotland, said: “One major incident affecting drinking water quality occurred during June 2015 when over 6,000 properties in North Lanarkshire suffered restrictions on the use of their water for two days after contamination of the supply.
“We have investigated this event thoroughly and submitted a report to the Procurator Fiscal.”
A spokesman for Scottish Water responded: “This matter is now with the Procurator Fiscal and we cannot comment further.
“Providing safe, wholesome drinking water to our five million customers is Scottish Water’s top priority.”
The report showed the company’s compliance with drinking water standards reached a record high in 2015.
Out of 308,356 samples of water taken from consumers’ taps, 99.92 per cent met the required standards.
Of the 35 incidents that were investigated, only the one in this area last June was classified as major.
Ms Petch said: “Consumers in Scotland are generally more satisfied with the taste and appearance of their tap water.
“Last year, only 0.2 per cent of consumers reported concerns with the quality of their supply, almost half the number from six years ago.”