The Lisbon-based European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) said 49 new "psychoactive" substances were officially notified for the first time in 2011 through an EU early-warning system.
"This represents the largest number of substances ever reported in a single year, up from 41 substances reported in 2010 and 24 reported in 2009," said the agency.
The largest group ? 23 ? were synthetic laboratory-designed substances that imitate the effects of cannabis, such as mephedrone marketed as Spice, and a further eight that imitate the effects of amphetamine and ecstasy.
The list of new substances also, for the first time, includes "designer medicines" which are synthesised to mimic the effects of known medicines by slightly altering their chemical structure.
The report also reveals that there has been an explosion in the number of online shops marketing legal highs over the past year.
It says the number of websites selling at least one synthetic drug has more than doubled from 314 in January 2011 to 690 in January this year.
"New drugs have become a global phenomenon which is developing at an unprecedented pace," says the EMCDDA report.
"The speed at which new drugs appear on the market challenges established procedures for monitoring, responding to and controlling the use of new psychoactive substances."
Wolfgang G?tz, the agency director, said: "We now see drugs marketed in attractive packages on the internet or sold in nightclubs and street corners.
"Whatever the source, the simple fact is that a dangerous game of roulette is being played by those who consume an ever-growing variety of powders, pills and mixtures without accurate knowledge of what substances they contain and the potential health risk they pose."
The agency says recent surveys show that across Europe, 5% of young people aged 15-24 say they have used legal highs.
In Britain, Latvia and Poland this rises to 10% and in Ireland it reaches 16%.
The UK's Serious Organised Crime Agency (Soca) told the EU that the extent to which organised criminals were involved in the trade in new drugs was unclear.
Roger Howard of the UK drugs policy commission, an independent organisation providing drugs policy analysis, said that when the 1971 Misuse of Drugs Act was passed new drugs were appearing once every few years whereas now they were being marketed almost once a week.
"We have rapidly growing numbers of psychoactive drugs on the market, and it's becoming increasingly difficult for the police to identify the drugs they're finding," he said.
"Just adding a drug to the long list already controlled won't make much difference.
"The police and forensics are under too much pressure already to be able to offer much deterrent to potential users.
"We are deluding ourselves if we think that using existing controls like temporary bans will solve the problem."