The relentless flood of new legal highs on to the market means it is "unfeasible" that they can all be controlled, a major report has concluded - weeks before a blanket ban comes into force in Britain.
It is unlikely that any regulatory system can be designed to "sufficiently limit" the stream of psychoactive substances without banning "a huge range of chemicals", an official study from European drug monitoring and law enforcement bodies warned.
The findings will bring fresh scrutiny on the Government's crackdown on so-called designer drugs - officially known as new psychoactive substances (NPSs).
Last week it emerged that new laws that prohibit the production, distribution, sale and supply of the substances have been delayed until May.
They had been expected to come into force from Wednesday but the start date was pushed back. Plans to include poppers in the ban were abandoned after official advisers said the drug does not fall within the scope of the current definition of a "psychoactive" substance in the legislation.
The new Act states that a substance produces a psychoactive effect "if, by stimulating or depressing the person's central nervous system, it affects the person's mental functioning or emotional state". A number of substances are exempt.
The 192-page EU Drug Markets Report released by the European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) and Europol on Tuesday said there are "no signs of a slowdown" in the development of NPSs.
Some 100 new substances were detected last year, bringing the number being monitored to 560. It means there are now more than twice as many new substances on the market as there are drugs controlled under international conventions.
The report concluded: "Finally, given the nature of the market and the continuous stream of new substances, it is unfeasible that all of them can be controlled.
"It is unlikely that any regulatory system can be designed to sufficiently limit the stream of new substances being manufactured without resorting to a ban on a huge range of chemicals."
Many NPSs are sold as legal replacements for banned drugs such as cannabis, heroin, cocaine and MDMA.
A Home Office spokesman said: "We refuse to allow the sale, production and supply of psychoactive substances in this country.
"These are dangerous drugs which, in 2014 alone, were involved in the tragic deaths of 144 people in the UK.
"This report recognises many of the challenges involved in taking effective action against this evil trade, which is costing lives and harming communities.
"We have worked tirelessly, after taking the advice of leading experts, to ensure that those challenges will be overcome by the landmark Psychoactive Substances Act."
For the first time the report gave an estimate of the size of Europe's drug market - putting the value at at least £19 billion a year.
The study also said there is evidence of links between organised crime groups involved in drug trafficking and terrorist organisations, although the relationships are "largely functional in nature".
The "fragmented nature" of terrorist activity in Europe "gives rise to concerns that groups or individuals with a dual criminal/terrorist profile may be harder to detect, as their activities may not register as important with either those concerned with organised crime or those investigating terrorism", it added.
Europol director Rob Wainwright said: "Illicit drug production and trafficking remains one of the largest and most innovative criminal markets in Europe.
"As it grows more complex and becomes entwined with other forms of crime, and even terrorism, it represents a key threat to the internal security of the EU."