Almost half of 15 and 16-year-olds admit to using cannabis, according to a United Nations (UN) report.
Up to a third of adults in the UK have also admitted taking cannabis, and critics say the decision to move the drug to class C in 2004 is to blame.
The Government announced that cannabis would return to class B last year, but some have raised concerns that penalties are still too soft.
According to the UN report, 44 per cent of UK youngsters aged 15 and 16 have used cannabis. This is up from 38 per in 2003 and puts the UK at the top of the league in Europe.
Shadow home affairs minister James Brokenshire said: “Topping the European league table for teenage cannabis is a shocking indictment. The health risks of psychosis, paranoia and long term mental health issues are becoming ever clearer and we are storing up significant problems for the future.”
Professor Hamid Ghodse, president of the International Narcotics Control Board, said that weakening the law in 2004 had “given the wrong message to young people in the UK and around the world”.
Other evidence has shown that there are now more than 22,000 people a year, almost half under the age of 18, being treated for cannabis addiction. In 1997 the number was 1,600.
The availability of skunk – the strongest form of cannabis – soared when the law was weakened. Cannabis-related admissions to mental hospitals have risen by 85 per cent since Labour came to power according to Government figures.
Despite a tougher law on cannabis coming into effect last month, magistrates warn that mixed messages are being sent because the penalties are not strict enough.
Cannabis users will not be arrested until they have been caught three times – a softer approach than is usually taken with other class B drugs. The first offence will result in a warning, and the second will incur an £80 fine, called a Penalty Notice of Disorder.
Days after the change in law, it emerged that teenagers in the North East of England had been given a booklet entitled Know Cannabis which tells readers that the drug can make music “sound better” and explains how to roll a joint.