EU officials concluded that, following a three-year investigation, there was no evidence to prove the previously undisputed fact.

Producers of bottled water are now forbidden by law from making the claim and will face a two-year jail sentence if they defy the edict, which comes into force in the UK next month.

Last night, critics claimed the EU was at odds with both science and common sense. Conservative MEP Roger Helmer said: “This is stupidity writ large.

“The euro is burning, the EU is falling apart and yet here they are: highly-paid, highly-pensioned officials worrying about the obvious qualities of water and trying to deny us the right to say what is patently true.

“If ever there were an episode which demonstrates the folly of the great European project then this is it.”

NHS health guidelines state clearly that drinking water helps avoid dehydration, and that Britons should drink at least 1.2 litres per day.

The Department for Health disputed the wisdom of the new law. A spokesman said: “Of course water hydrates. While we support the EU in preventing false claims about products, we need to exercise common sense as far as possible."

German professors Dr Andreas Hahn and Dr Moritz Hagenmeyer, who advise food manufacturers on how to advertise their products, asked the European Commission if the claim could be made on labels.

They compiled what they assumed was an uncontroversial statement in order to test new laws which allow products to claim they can reduce the risk of disease, subject to EU approval.

They applied for the right to state that “regular consumption of significant amounts of water can reduce the risk of development of dehydration” as well as preventing a decrease in performance.

However, last February, the European Food Standards Authority (EFSA) refused to approve the statement.

A meeting of 21 scientists in Parma, Italy, concluded that reduced water content in the body was a symptom of dehydration and not something that drinking water could subsequently control.

Now the EFSA verdict has been turned into an EU directive which was issued on Wednesday.

Ukip MEP Paul Nuttall said the ruling made the “bendy banana law” look “positively sane”.

He said: “I had to read this four or five times before I believed it. It is a perfect example of what Brussels does best. Spend three years, with 20 separate pieces of correspondence before summoning 21 professors to Parma where they decide with great solemnity that drinking water cannot be sold as a way to combat dehydration.

“Then they make this judgment law and make it clear that if anybody dares sell water claiming that it is effective against dehydration they could get into serious legal bother.

EU regulations, which aim to uphold food standards across member states, are frequently criticised.

Rules banning bent bananas and curved cucumbers were scrapped in 2008 after causing international ridicule.

Prof Hahn, from the Institute for Food Science and Human Nutrition at Hanover Leibniz University, said the European Commission had made another mistake with its latest ruling.

“What is our reaction to the outcome? Let us put it this way: We are neither surprised nor delighted.

“The European Commission is wrong; it should have authorised the claim. That should be more than clear to anyone who has consumed water in the past, and who has not? We fear there is something wrong in the state of Europe.”

Prof Brian Ratcliffe, spokesman for the Nutrition Society, said dehydration was usually caused by a clinical condition and that one could remain adequately hydrated without drinking water.

He said: “The EU is saying that this does not reduce the risk of dehydration and that is correct.

“This claim is trying to imply that there is something special about bottled water which is not a reasonable claim.”

In 2004, EU authorities claimed Boeing received $19 billion in unfair subsidies from federal and state governments between 1989 and 2006.

The U.S. government filed a similar claim the same year against Germany, France, the United Kingdom, Spain and the EU over subsidies given to Airbus, owned by European Aeronautic Defense & Space Co..

According to Monday's ruling, the WTO's appellate body said Boeing received between $3 billion and $4 billion in U.S. subsidies, according to Kirk.

By contrast, the WTO said in December that Airbus received $18 billion in subsidies from European governments.