Environmental campaigners have attacked Starbucks after the discovery that millions of litres of water are wasted in its coffee shops every day, contradicting its much-boasted green credentials.

An investigation by the Sun revealed that over 23.4m litres of water are poured down the drains of 10,000 outlets worldwide due to a policy of keeping a tap running non-stop.

It is enough daily water for the entire 2 million population of Namibia in Africa, which has severe droughts, or to fill an Olympic pool every 83 minutes.

A single Starbucks tap left running for just over three minutes wastes the amount of water one African needs to survive for a day in drought conditions.

Each Starbucks has a cold tap behind the counter that runs into a sink known as a "dipper well" - used to wash utensils.

Under the company's health and safety rules, staff are banned from turning the water off because management claim that a constant flow of water prevents germs breeding in taps.

Water companies joined green activists in criticising the firm for harming the environment and wasting a vital natural resource. Experts said leaving taps running for hygiene reasons was "nonsense".

Water shortage is one of the world's biggest problems. Australia is in the grip of a seven-year drought - the worst in a century.

In the UK, Starbucks has 698 branches, each open for 13 hours a day. Even a slow tap flows three litres of water a minute, meaning Starbucks in the UK is wasting an estimated 1.63m litres a day – enough to supply Matlock village in Derbyshire.

The running water policy was revealed after a Starbucks executive wrote back to a couple who complained about the tap at their local branch.

Lisa Woolfe, 39, of Cuffley, Hertfordshire, said: "I noticed a small sink behind the counter had its tap running. The assistant said the store was told to keep it running as it cleaned the pipes.

"I could not believe it but when we contacted head office, they confirmed the taps were left on and the water was not recycled.

"It is an absolutely astonishing waste of water, especially for a company which prides itself on its green credentials."

Speaking to staff at Starbucks outlets around the world, the Sun found that many did not use the running tap or even know what it was for.

Peter Robinson, of the 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.

"Tap water comes from rivers and groundwater and wasting it can cause great harm to the environment and wildlife. Big companies should set an example."

Jacob Tompkins, of the independent water efficiency agency Water Wise, said: "If they are doing all their basic cleaning procedures, I fail to see why they would need to do this. There are a lot of other ways to stop a build-up of bacteria.

"The chance of a build-up in the spout is extremely remote. And if there is one they're not cleaning the tap properly."

Ian Barker, the head of water resources at the Environment Agency, said: "We are already taking too much water from the environment and are seeing reduced river flows."

A spokeswoman for Starbucks said that the company's water use adhered to the World Health Organisation, US and EU environmental directives for in-store water supply standards. But she acknowledged the company could cut its water use.

She defended its dipper well system, saying the technique was common and accepted in the industry.

"Starbucks' challenge is to balance water conservation with the need for customer safety," the spokeswoman said. "The dipper well system currently in use in Starbucks retail stores ensures that we meet or exceed our own and local health standards."

She said the company had tested alternative methods such as the use of an ice bath but it was not successful. It was considering other alternatives and cut its water use per square foot this year.

Starbucks is known for its campaigns and instore advertising boasting how it gives back to communities and the environment.

In the company's latest corporate social responsibility report, it says: "From promoting conservation in coffee-growing countries to in-store 'green teams' and recycling programs, Starbucks has established high standards for environmental responsibility.

"By taking steps to reduce waste from our operations and recycle, we can preserve the Earth's natural resources and enhance the quality of lives around the globe.

"Starbucks actively seeks opportunities to minimise our environmental impact."

This is not the first time the Seattle-based firm has come under fire over its social and environmental credentials. In 2006, the Guardian reported how the US coffee giant has used its muscle to block an attempt by Ethiopia's farmers to copyright their most famous coffee bean types, denying them potential earnings of up to £47m a year.

As a result, Starbucks negotiated an agreement with the Ethiopian government to give the country more ownership and a better price for its coffee beans.