An investigation has been launched after drinking water on board 14 Cathay Pacific aircraft – around 10 per cent of its entire fleet – was found to be tainted.

Hong Kong’s Port Health Office (PHO) collected samples from 22 planes, as part of a routine examination, earlier this month. Tests found that 14 of the samples failed to meet minimum hygiene standards.

According to the airline, nine of the 14 affected aircraft have had their water tanks cleaned and disinfected, and the results of follow-up tests are pending. Tanks on the five other affected jets will be cleaned imminently. Hong Kong Aircraft Engineering Co, which transports water from supply stations to the aircraft, has cleaned all of its 11 vehicles.

In the meantime Cathay Pacific is urging passengers not to brush their teeth in the plane lavatories and is handing out bottled water to everyone on board.

“We have reviewed the overall water collection and filling procedures for aircraft and investigations so far have not identified the source of contamination,” said a spokesman for Hong Kong’s Department of Health.

Cathay Pacific has water samples collected from each of its aircraft every six months, while all water tanks are cleaned and disinfected every three months.

It was named the world’s 4th cleanest airline, behind EVA Air, Singapore Airlines and ANA All Nippon Airways, at the most recent Skytrax World Airline Awards, based on a survey of 18 million passengers.

While airlines routinely serve free tap water, the same water used for tea and coffee, several investigations have suggested that supplies are prone to contamination. A Wall Street Journal study in 2002 saw samples taken on 14 different flights, with some worrying findings.

“The results of our water-quality snapshot: a long list of microscopic life you don’t want to drink, from Salmonella and Staphylococcus to tiny insect eggs,” it said. “Worse, contamination was the rule, not the exception: Almost all of the bacteria levels were tens, sometimes hundreds, of times above U.S. government limits.”