Britons on average are taking more legal-high drugs than their peers in any other EU country, according to a report by the United Nations.
According to the World Drug Report, there are 670,000 people under the age of 25 in Britain who consume substances that have been altered in laboratories to evade UK drug laws.
In total, consumption of legal highs in the UK accounts for almost a quarter of the total across 27 European countries, said the UN Office on Drugs and Crime.
It called the findings alarming and said that they posed unforeseen health challenges.
But UK charity Drugscope cast doubt on the figures, claiming they included use of banned substances such as mephedrone.
Harry Shapiro, of Drugscope, told IBTimes UK: "The figures are misleading.
"There are no figures on the number of people who use legal highs, as they are by definition legal. I think the UN looked at the crime survey and saw a figure which relates to things like mephedrone which are already illegal.
"They seem to mash up legal highs and new psychoactive substances which includes mephedrone, synthetic cannabis and Benzylpiperazine."
He warned that a growing number of young Britons were encountering problems from using legal highs.
"We've got a drug scene in transition," Shapiro said. "It's clear that heroin, crack and ecstasy use has been coming down, but alongside that there are these other chemicals.
"There ares unknown numbers of young people trying them and getting in to difficulties because these things are more toxic."
Drugscope had witnessed increasing numbers of legal high users being taken to hospital, he said.
The easy availability of legal highs is revealed by a quick search online with sellers offering a plethora or substances.
Charity Addaction said that young people were at even more risk from substances bought online than from street dealers.
"Legal highs are predominately sold on the web as things like 'plant food'," said a spokesman.
"The difference is drug dealers on the street want you to come back to them and buy again. Whereas to these companies online in places like south Asia, you are only a number on a screen."
Experts warn that some legal-high substances such as synthetic cannabis could be more dangerous than their natural alternatives.
By tinkering with the chemical makeup, technicians often remove the natural "safety valve" some drugs have. Altering a substance's makeup also side-steps the law, by placing it outside of parametres which define a substance legally.
As a result, a naturally occurring ingredient in cannabis which acts to counteract the high is often absent from its lab-grown counterpart, heightening further the risk of harm.