Two in five 15-year-olds has in the UK has tried cannabis, which is more than anywhere else in Europe, figures show.
Most users are young men
About one in 10 of this age group have smoked pot at least 40 times in the last year, says the European Monitoring Centre for Drugs and Drug Addiction.
The UK also has the joint highest number of young cocaine users, alongside Spain.
The Home Office said the data was two years old and did not reflect current levels of use, which are much lower.
The report comes as ministers announced harsher sentences for dealers caught selling near schools.
The EMCDDA found 42% of boys and 38% of girls aged 15 in England had tried cannabis.
Looking at the 25 European Union member states, up to 3m people could be using cannabis each day across Europe, particularly young men.
After cannabis, ecstasy is rivalling amphetamines as the second most popular drug, with the highest levels of consumption in Britain, where 12 percent of all adults have popped at least one pill.
UK topping league
EMCDDA director Georges Estievenart said: "We have established that regular cannabis use is rising.
"We know from clinical studies that cannabis users can
experience acute and chronic health problems."
He said more information was needed to gauge what affect long-term use was having on users.
But Home Office minister Caroline Flint said more recent trends have seen a steady fall in the use of cannabis,
amphetamines and ecstasy.
"The British Crime Survey shows that amphetamine use by 16- to 24-year-olds has fallen by 60% in the last five years, cannabis use has declined from 28.2% in 1998 to 24.8% today and crack and cocaine use has stabilised.
"Drug use is still too high and we are planning new legislation aimed at getting more users into treatment - including testing on arrest - and strengthening police powers to tackle drug dealers."
Home Secretary David Blunkett announced new measures to stop dealers selling drugs near schools.
One of the proposals is to make it an aggravating factor on sentencing for a dealer to sell drugs near schools or use children as couriers.
Under a new Drugs Bill, there would also be a presumption where people are found with larger quantities of drugs that they intend to supply - a far more serious offence than possession.
Those arrested for some offences would face a compulsory drugs test to get more addicts into treatment.
Liberal Democrat home affairs spokesman Mark Oaten said: "These are constructive proposals, but with no date for a Bill they could fall by the wayside before the election."
Shadow home secretary David Davis said the figures were evidence that government's decision to downgrade cannabis from Class B to Class C last January was wrong.
"As this report clearly shows Labour have failed to get a grip on spiralling drug use.
"Drugs are a scourge on society yet the Government have put out mixed messages on cannabis."
Martin Barnes, chief executive of DrugScope, said: "It is concerning to see young people using drugs, but we should distinguish between problem use of drugs and young people experimenting, often on a one off basis.
"Whilst there is no room for complacency, it is important to acknowledge that the vast majority of young people do not use drugs at all.
"We need to ensure that young people have the right information to make informed choices as scare campaigns or criminal sanctions alone do not work."
And the UK is among the top five countries for "problem drug users", requiring medical help, reporting six to 10 cases for every 1,000 adults, along with Denmark, Italy, Luxembourg and Portugal.
Latest figures from the Department of Health for 2003 show almost a third (30%) of youngsters aged 11 to 15 have ever taken illegal drugs, up from 27% in 2002.
Of these 12% had taken drugs in the last month before the survey and 21% in the last year.