An investigation has revealed that every Starbucks branch has a cold tap behind the counter providing water for a "dipper well" sink used for washing spoons and utensils, but staff are banned from turning the water off under health and safety rules.
The policy covers all of its 10,000 outlets worldwide, meaning that 23.4m litres of water are wasted every day.
In the UK alone, where there are 698 branches open 13 hours a day, around 1.63m litres of water is being wasted.
Starbucks has hyped up its green credentials in the past and claims to actively seek opportunities to minimise its environmental impact.
A spokesman for UK Water, which represents UK water and waste water service suppliers, said that there was no need to leave taps running and that it was "wasteful and unnecessary".
Peter Robinson, who works for environmental charity Waste Watch, said: "Leaving taps running all day is a shocking waste of precious water. And to claim you are doing it for health and safety reasons is bonkers."
Starbucks claims that dipper wells use a stream of continuous cold fresh-running water to rinse away food residue, help keep utensils clean and prevent bacterial growth, and that the technique is common and well-accepted in the industry.
However the coffee chain did admit that it could be doing more to reduce its water usage. A spokeswoman for Starbucks said: "Although our current water usage adheres to the World Health Organisation, US Environmental Protection Agency and the European Union directives for all in-store water supply standards, we recognise the opportunity exists to reduce our total water usage."