The results suggested cocaine was in regular use in the areas tested

The River Po was found to be carrying the equivalent of nearly 4kg (8.8lb) of cocaine daily. The Po Valley is home to about five million people.

The study estimated daily consumption to be about 27 doses (100mg or 0.004oz each) per 1,000 young adults.

The study was published by the web journal Environmental Health.

The chemical tested - benzoylecgonine (BE) - had arrived via the sewage system from the urine of drug users. A by-product of cocaine metabolism, it cannot be produced by other means.

The estimated daily consumption "greatly exceeds official national figures," the report says.

'Staggering' figures

The team says the test has to be refined before being applied on a vast scale.

The team, led by Dr Ettore Zuccato from the Mario Negri Institute for Pharmacological Research in Milan, said they were taken aback by their findings, whose economic impact they described as "staggering".

"The large amount of cocaine (at least 1,500 kg/3,307lb) that our findings suggest are consumed per year in the River Po basin would amount... to about $150m (£84m) in street value," the scientists wrote in the journal.

They said the figure of 4kg of cocaine equivalent carried by the Po daily indicated about 40,000 doses of cocaine per day for the region.

But official estimates indicated only about 15,000 doses per month for young adults in the region.

The team hope that in the near future, they will be able to develop similar tests in order to assess the true number of cannabis and heroin users.