Producers of bottled water are now forbidden by UK law from claiming their products will prevent dehydration after a three-year investigation from EU officials.
Britain's The Telegraph has reported the decision was made on the basis there was no evidence to prove the previously undisputed fact.
Producers of bottled water 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 the law was "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," he told The Telegraph.
"If ever there were an episode which demonstrates the folly of the great European project then this is it."
Britain's National Health Service guidelines clearly state 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.
"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," a spokesman said.
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.
The Nutrition Society spokesman Prof Brian Ratcliffe said dehydration was usually caused by a clinical condition and that one could remain adequately hydrated without drinking water.
"The EU is saying that this does not reduce the risk of dehydration and that is correct," he said.
"This claim is trying to imply that there is something special about bottled water which is not a reasonable claim."