The watchdog said it had immediately conducted an in-depth study and follow-up after receiving the company’s legal letter and meeting its representatives.

“We apologise for the misunderstanding caused by the discrepancy in sample categorisation in this test,” the council said.

“We understand that the product is not ‘natural mineral water’ nor ‘purified drinking water’. It is ‘natural drinking water’.”

The council said the sample of Nongfu Spring’s bottled water had been re-rated and reclassified as an independent category of “natural drinking water”.

It said the sample, in which three micrograms of bromate were found per litre, fell within the “drinking water” range under EU safety standards for that category.

The change in category meant the relevant maximum level of bromate under the EU standard was 10 micrograms per litre – not three micrograms as previously referenced – and the product was in line with the mainland’s safety standard for packaged drinking water.

“After the re-rating, the overall performance of the sample was adjusted from 4½ stars to five,” the council said.

Nongfu Spring said it noted the council’s clarification.

“The testing results show that products from Nongfu Spring fully comply with relevant standards and can be consumed safely,” the company said.

The bottled water brand held 26.5 per cent of the mainland’s drinking water market share last year, the largest of any company. As of Thursday, it had a market cap of HK$397.56 billion (US$50.92 billion).

Founded by country’s richest man, Zhong Shanshan, Hong Kong-listed Nongfu Spring stock dipped to a low of HK$32.20 on Tuesday, down from Monday’s opening price of HK$34.95.

After the watchdog’s apology on Thursday, its stock price rose to HK$35.90 before closing at HK$35.35.

Bromate is formed when ozone used to disinfect drinking water reacts with naturally occurring bromide found in source water.

After testing 30 bottled water products sold in the city earlier, the council on Monday claimed that Nongfu Spring and Ganten – both mainland brands – contained bromate that hit the EU limit of three micrograms per litre for natural mineral water.

The EU safety standard for bromate in drinking water, in contrast, is set at 10 micrograms a litre.

The report also cited the International Agency for Research on Cancer, which had classified one type of bromate, potassium bromate, as possibly carcinogenic to humans.

The product should have been assessed against the EU drinking water safety standard, not the natural mineral water one, it said.

The company added that the council had applied incorrect standards when evaluating the product and misled the public through subjective misrepresentation.

The council said on Thursday that when it conducted a test on the samples late last year, it found that 29 of them were clearly labelled as to their category of water.

“Nongfu Spring was labelled as natural drinking water,” its spokesman said.

“Based on the four types of mineral indicated on its list of ingredients, which was similar to that of natural mineral water, our staff classified [the product] as natural mineral water, according to the sample’s name and labelling information.”

The council said that while testing standards for products were typically based on their jurisdiction of origin, the nature of testing for the study had been comparative, which meant tests and assessments were based on international standards.