Green groups branded the practice "ridiculous", claiming it suggests a lack of faith in Northern Ireland's water system and undermines the morale of the people who work there.

Responding to a written question in the House of Commons this week, Mr Woodward revealed the NIO spent £33,665 on the purchase of bottled water in 2006/07 — the last year for which figures are available. This sum included the supply of the bottles of water used in water cooler machines.

A recent BBC Panorama documentary outlined the shocking environmental cost of choosing bottled water, revealing that a litre of the UK's most popular French mineral waters generates up to 600 times more CO2 equivalent than a litre of Thames tap water. Meanwhile, sales of bottled water have grown 200-fold since the 1970s.

Green Party Assembly member Brian Wilson said spending £33,000 on bottled water was an "awful waste of money"."Obviously it's ridiculous. In the Assembly we are trying to reduce the use of bottled water," he said.

Mr Wilson said each member of the Regional Development Committee used to be provided with two bottles of water, but that has now been replaced with a jug.

"I drink ordinary tap water — there's nothing wrong with it and I've never drunk bottled water in my life. The environmental impact of that is quite significant when we can get our local water from the Mournes which is more beautiful than any French water," he said.

"This goes far, far beyond the civil service and public bodies. People seem to think the bottled water is better for them, but it's a waste of money and the environmental impact is unacceptable."

Mr Wilson said the practice doesn't say much for the NIO's faith in the water service.

"It's undermining the morale of the people who work in the water service," he said.

Friends of the Earth said it was ridiculous that so much money is spent on bottled water for civil servants.

"If our politicians were serious about sustainable development, combating climate change and tackling waste they would stick to tap water," campaigner Declan Allison said.

"We expect our elected representatives to show leadership, and switching from wasteful bottled water to tap water would be a simple and positive step."

An NIO spokesperson said: "The NIO is moving towards a policy of eliminating the use of bottled water."

Earlier this week, the DUP's Sammy Wilson joined with party colleague Jim Wells to back a motion which "notes the environmental impact of using bottled water and calls upon the Assembly Commission to end the use of bottled water at committee meetings and Assembly functions, recognising that tap water in Northern Ireland is safe, drinkable and economic; and believes it is important for the Assembly to set an example in reducing the waste involved in bottling and transporting bottled water."