Cocaine use in the UK is now so common that traces of the drug can be found in our drinking water, tests show.

Inspections of tap water at four different sites found a metabolised form of the illegal drug, which showed it had already passed through the human body.

The levels were so low that they posed no danger to health, but come as a startling indication of how widespread drug use has become.

Well-publicised tests in the past have found traces of cocaine on nearly every banknote in circulation, in toilets in the House of Commons and at two thirds of Cambridge colleges.

But being able to find traces of cocaine in tap water, even after stringent purification processes, demonstrates how common the drug has become.

Benzoylecgonine, the form of cocaine that is generated once the drug has been processed by the body, was found in tests at four sites by the Drinking Water Inspectorate.

It is the same compound that is searched for in urine-based drug tests for cocaine.

Steve Rolles, from the drug policy think tank Transform, told The Sunday Times that the findings were an indication of the scale of the use of the drug in Britain today.

‘We have the near highest level of cocaine use in western Europe,’ he said.

‘It has also been getting cheaper and cheaper at the same time as its use has been going up.’

Cocaine is the only major drug for which use has increased overall since 1996, with its falling price thought to be a major reason for its prevalence.

Now, the drug costs around £40 per gram in Britain, compared to as much as £115 in the U.S..

While in the 1980s and 1990s it was seen as a drug of the wealthy and fashionable, it is now widely taken by people of every class and profession - and even by schoolchildren.

The UN Office on Drugs and Crime estimates that one in 20 British teenagers aged 15 and 16 have tried cocaine.

The number of people in treatment for cocaine addiction in the UK rose from 10,770 in 2006-07 to 12,592 in 2007-08 - and nearly 700,000 people aged 16 to 59 are estimated to take cocaine every year.

A report by St George’s University London, published in February, found that cocaine killed 115 people in Britain in 2012, of 1,700 drug-related deaths.

As well as benzoylecgonine, the drinking water tested also contained significant quantities of caffeine.

Traces of the common pain-killer ibuprofen and carbamazepine, a drug for treating epilepsy, were also discovered.

A report from Public Health England published in September 2013 analysed the Drinking Water Inspectorate’s results and concluded there was nor risk to the public.

‘Intakes of the compounds detected in drinking water are many orders of magnitude lower than levels of therapeutic doses,’ the report said.

‘Estimated exposures for most of the detected compounds are at least thousands of times below doses seen to produce adverse effects in animals and hundreds of thousands below human therapeutic doses.

‘Thus, the detected pharmaceuticals are unlikely to present a risk to health.’

Sue Pennison, principal inspector at the Drinking Water Inspectorate, said: ‘The study which looked at “worst case” scenarios is reassuring in that it demonstrated that water treatment was generally very effective in the removal of a number of pharmaceuticals which were detected in untreated river water in trace amounts.

‘Only six compounds were detected in treated river water and advice on the results received from Public Health England concluded that exposures for the detected compounds was of the order of thousands of times below the level associated with adverse effects in animals and hundreds of thousands of times below human therapeutic doses.’

She added: ‘The study contributes to the update of national risk assessments of water supplies which is an ongoing activity.’