Cocaine use has more than doubled in Britain in seven years, with the purity of the drug hitting a record high, an analysis of wastewater has revealed. Scientific tests of the metabolised drug in sewage suggest that in London, users are collectively taking almost 200,000 doses of the drug every day, worth some £700,000 ($NZ136,4700) daily at current street prices.

The analysis by forensic scientists at King's College London shows concentrations in wastewater have risen from 392 milligrams per 1000 of the population per day in 2011, to 900 milligrams per 1000 now. The average dose through snorting a line or smoking is 40 milligrams, which would mean that the equivalent of more than one in every 50 people in the city's population of nine million is taking the drug daily.

London is also one of the few cities in Europe where consumption of the drug is almost as high during the week as at weekends. Similar tests of Bristol's sewage have seen concentrations of cocaine almost quadruple from 248 milligrams of the drug per 1000 of the city's population in 2014 to 969 in 2018. Both cities are among the top five in Europe alongside Barcelona, Antwerp, Zurich and Amsterdam.

Dr Leon Barron, a forensic scientist at King's College who led the research, said the analysis provided a comprehensive, real-time assessment of how much cocaine was being consumed by the population and its purity.

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"It's been steadily rising. The obvious reason is that there is an increased consumption by the population. I understand that purity has also risen mainly through increased supply and production in Latin America. There are cartels operating in the UK to offload that excess supply," he said.

The analysis is confirmed by the latest assessment by the National Crime Agency (NCA), which says cocaine purity is at "historically high levels" and that there is "evidence of a resilient supply chain" servicing the market for the drug in the UK. Police chiefs led by Cressida Dick, the Scotland Yard commissioner, have accused middle-class cocaine users of "having blood on their hands" for fuelling gang violence and knife crime through their habits.

Figures provided for The Sunday Telegraph by the NCA show its cocaine seizures have trebled in five years, from 42.8 tons in 2013/14 to 122.9 tons in 2017/18.

"Multi-tonne shipments [of cocaine] to the UK have become increasingly common," it said.

It also warned that there were indications of increased use of crack cocaine, which has started to take hold among some university students.

Cocaine purity has risen to 80 per cent plus, said Trevor Shine, director of Tictac at St George's Hospital in London, which analyses illegal drugs. Cocaine of that purity commands £100 per gram on the street.

He said one reason for the purity was that gangs, primarily Albanian, were importing directly from South America and were passing the savings on to the consumer rather than diluting the drug. Traces of cocaine have been found in fish, and in shrimps in Suffolk.

However, Dr Barron said sewage treatment was effective in removing most of the metabolised cocaine from wastewater before it was recycled into rivers.

From 3000 parts per trillion - equivalent to half a teaspoon of cocaine in an Olympic swimming pool - treatment reduced it to about two parts per trillion.

There was, therefore, no public health risk from cocaine in the water supply, he added.