A record 57 new legal highs have been detected so far this year, with the EU's early warning system reporting the appearance of more than one new psychoactive drug on the market every week.

The rise and rise of legal highs is being driven by an explosion in the number of online retailers selling the new drugs in Europe, which has risen from 170 in 2010 to a record 693 internet "head shops" identified in a snapshot earlier this year.

The rapid spread of the market in legal highs, partly driven by organised crime, is revealed in the annual report from the Lisbon-based European Monitoring Centre for Drugs and Drug Addiction published on Thursday.

The drug experts say that while cocaine, ecstasy and amphetamines continue to be the main players for those taking stimulants across Europe, they are now competing with a growing number of emerging synthetic drugs.

The EU's drug agency says nearly 10% of 15-to 24-year-olds in Britain have used a legal high at some time – twice the European average of 5% for this age group. This is exceeded only by the 16% recorded for this age group in Ireland and matched by similar levels in Poland and Latvia.

The annual survey also shows that the UK remains at the top of the European league table for all age groups for lifetime ecstasy use (12.4%) and amphetamines (12.9%), and second only behind Spain for cocaine use (12.8%), albeit at a lower level for young adults than in recent years.

But it says the UK has seen a substantial decline in the levels of cannabis use since 2008 and is no longer near the top of the table, with the Czech Republic, Spain, Italy and France recording the highest levels of use on a European scale.

Indeed the report says that the proportion of young British adults who have used cannabis in the last 12 months has now fallen to the EU average of 12.4%.

The report also says that traditional hard drugs are failing to find new recruits in Britain, with a significant decline in the numbers of people entering specialist drug treatment for the first time for heroin.

The annual EU drugs report says two-thirds of the newly reported legal highs on sale are synthetic drugs designed to imitate the effects of cannabis or cocaine.

It says most of the new psychoactive substances sold as legal highs and appearing on the European illicit market are synthesised outside Europe in China and, to a lesser extent, India. Organised crime groups are now reported to be involved in manufacturing the tablets and marketing them.

The annual European survey says the number of new psychoactive drugs being reported in the EU via its early warning system has risen from 24 in 2009 to 41 in 2010, 49 in 2011 and 57 so far this year.

The European drug experts say those manufacturing the drugs are trying to evade attempts to ban them by using ever more obscure chemical groups to synthesise them; a rising number of products now include several psychoactive substances.

Seizures and test purchases have revealed samples containing both legal and illegal substances.

Although three natural products – kratom, salvia and magic mushrooms – lead the top 10 legal highs most frequently offered online, the remaining seven places are all occupied by synthetic substances.

The EU 2012 snapshot survey identified a marked rise in the number of shops offering synthetic cathinones, which can mimic the effects of cocaine, suggesting that online operators are looking for a replacement for mephedrone, which has been banned in Britain and across the EU.

The UK has introduced a system of temporary banning orders, which are introduced pending a full examination of the harms associated with a new substance.

Cecilia Malmström, the European commissioner for home affairs, said she was struck by the speed of developments in this area.

"Stimulant and synthetic drugs play a central role in the European drug situation, creating a market that is fast moving, volatile and difficult to control," she said.

"More than ever before young people are being exposed to a plethora of powders and pills.

"I think it is clear to all that strong and co-ordinated actions are required if we are to respond effectively in this area."