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.”

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