Scientists say London has the highest concentration of cocaine in sewage of anywhere surveyed in Europe.

The data from the European Union's drug monitoring body found the capital slightly ahead of Amsterdam.

While London comes top for cocaine flushed down the toilet, Amsterdam's drains contain greater amounts of cannabis.

The European Monitoring Centre for Drugs and Drug Addiction data comes from sampling in more than 50 cities.

The results, which take into account the size of each city's population, show that on average, drug users in London relieved themselves of 737mg of cocaine per 1,000 people during the week in 2014.

The amount of cocaine in London's waste water peaks on a Friday and Saturday, but then begins to fall away dramatically on Sunday and into Monday.

Flushed away Cocaine in European waste 737mg Average daily amount of cocaine per 1,000 people found in London sewage during 2014 716mg in Amsterdam

632mg in Antwerp

0.2mg Smallest amount found in Jvaskyla in Finland

The amount of cocaine found in both Antwerp and Amsterdam continues to rise into Sunday, potentially suggesting that the drug's peak consumption in these cities comes or continues later into the weekend than in the UK. A detailed analysis of weekend waste water shows that Amsterdam ranks higher than London over Saturday and Sunday alone.

Scientists around the world have been increasingly monitoring waste water in cities so they can draw a more accurate picture of drug consumption over the year.

The figures for London tally with the monitoring agency's wider research which indicates that the UK has the highest rate of cocaine use among young adults in Europe, It said that around 4% of people aged between 15 and 34 had said they had taken the drug in the 12 months leading up to the 2013/14 report. While there are fluctuations from year to year, most studies indicate that most drug use is in decline.

The scientists tested for five different drugs and found that Amsterdam came top of the league table for both ecstasy and cannabis. Oslo in Norway and Dresden in Germany had by far the highest amounts of methamphetamine in sewage - while London had no trace at all.

The annual 2015 report from the EMCDDA warns that while there were hundreds of websites openly selling so-called "legal highs", drug agencies had a poor understanding of the trade on the dark web - the parts of the internet which are not accessible through standard searches.

"Cryptocurrencies, like Bitcoin, are used to facilitate anonymous transactions, and stealth packaging is used to facilitate transportation of small quantities of drugs through established commercial channels," said the report.

"Evidence suggests that many illicit drug purchases made on the deep web are intended for resale. Another development relates to drug supply and the sharing of drugs or drug experiences via social media, including mobile apps. This area remains both poorly understood and difficult to monitor."

"Together, the growth of online and virtual drug markets poses major challenges to law enforcement and drug control policies. The fact that manufacturers, suppliers, retailers, website-hosting and payment processing services may all be based in different countries makes online drug markets particularly difficult to control."