Britain is the cocaine capital of Europe with more than one million regular users of the drug, according to a United Nations report.
Its findings reveal that 5 per cent of 16 year-olds have used cocaine, undermining government claims that cocaine use is declining.
The UN Office on Drugs andCrime said enforcement actions had resulted in the purity of cocaine falling since 2006, at wholesale and retail levels.
The purity of cocaine seized by police fell from 32 per cent on average in 2007 to 23 per cent in the first quarter of 2009.
Purity levels of drugs seized by customs fell from 67 per cent to 56 per cent.
The UN said dealers were diluting cocaine with agents such as dental and veterinary anaesthetic that mimicked the effects of cocaine but was cheaper.
The dilution suggested the establishment of large international trafficking activities in cutting agents, which were usually legal substances.
Cocaine wholesale prices rose from £30,000 (NZ$77,000) a kilo in 2007 to £45,000 a kilo at the start of this year. Cocaine is shipped from the Caribbean or increasingly, from the west coast of Africa.
The report states that the UK "continues to be - in absolute numbers - Europe's largest cocaine market, with its second-highest cocaine use prevalence rate".
About 74,000 "problem drug users" live in London, meaning about one resident in 100 has a drug problem.
Europe's second-largest market is Spain, followed by Italy and Germany. The report also found that there was a "clear downward trend" in the use of cannabis among young Britons.
