Despite the presence of British forces, Afghanistan has produced two record opium harvests, with Helmand province accounting for more than half the poppies grown.
And the UK remains at the top of the European "league table" for cocaine abuse for the fifth consecutive year, according to the annual report of the European Monitoring Centre for Drugs and Drug Addiction.
But it reports that there are "stronger signals" of the declining popularity of cannabis across Europe, with a strong downward trend especially noticeable among British school students.
The agency suggests this decline may be a byproduct of the introduction of widespread bans on smoking tobacco in public places. Nevertheless the drugs experts say that a quarter of all Europeans - 71 million people - have tried cannabis at some point in their lives, 12 million of them within the last month.
AddAction's Andrew Horne on why Britain leads Europe in heroin and cocaine use
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The heroin warning follows two record opium harvests in Afghanistan of 8,200 tonnes in 2007 and 7,700 tonnes this year. These harvests represent 90% of the world's illicit opium production, with Helmand province, the centre of British military operations, accounting for more than half of the poppies grown. The agency says "alternative development" measures to persuade farmers to switch to other crops are having a very limited impact.
The EU is worried that these record harvests are threatening to bring to an end the "slowly improving" heroin situation in Britain and across Europe, and reverse the decline seen in heroin-related drug deaths. Seizures have doubled in Turkey, an important transit country, are up 10% up in Britain, and pockets of new injecting heroin users are appearing for the first time in recent years.
"Current evidence does not point to an epidemic growth in heroin problems as experienced by most of Europe in the 1990s," said the EU drugs agency director, Wolfgang Gotz. "Nonetheless, we cannot ignore the threat posed by the glut of heroin now available on the world market, the concerns raised by indicators of heroin use, or signs that synthetic opiates may be a growing problem. Vigilance is clearly required."
Britain's continuing position at top of the league table of 27 EU countries for cocaine abuse is based on the fact that 12.7% of young adults aged 15 to 34 have used the drug. Typical cocaine users in Britain are now just as likely to be poor working class young men as wealthy city traders. The latest school surveys show that 5% of teenagers aged 15 and 16 have tried the drug.
Cocaine use in Europe is concentrated in only a few countries. Use is highest in Britain and Spain but has stabilised in both countries in recent years at a level similar to that seen in the US. The increasing number of Europeans using the drug – an estimated 4 million last year - reflects its recent growth in Italy, Denmark and Ireland. The increasing popularity of cocaine has been matched by declining use of cannabis.
There is however some encouraging news about cannabis consumption in Britain. While the UK consistently had the highest levels of cannabis use among school children in the early and mid 1990s, in this European survey it has seen the sharpest decline in popularity of any EU country.
Britain is now fourth in the European league table for cannabis use amongst 15- to 24-year-olds, with 39.5% saying they have tried it and 12% saying they have used it in the last month.
The European drugs experts say domestic herbal cannabis production is no longer marginal in some EU countries, including Britain, with 2.3 million cannabis plants seized last year in Europe.
The most recent Eurobarometer poll on the subject shows increasing support for decriminalisation of cannabis, with 31% of Europeans backing the idea.