As well as phenacetin, a pharmaceutical painkiller banned in many countries for causing kidney failure and cancer, samples also contained a dental painkiller, a chemical used for de-worming pets, and boric acid -- which is used to kill cockroaches.

"The picture is not encouraging when it comes to cocaine," said Martin Barnes, chief executive of DrugScope.

"Prices have fallen and in some parts of the country, cocaine dealers are offering a heavily cut 'economy' version of the drug at £30 a gram as well as a less adulterated 'luxury' version for £50 a gram."

"It is a drug which has severe health risks."

Police analysis of samples reveals that cocaine's purity has plunged over the last few years.

"Five years ago, most of the street cocaine we seized had a purity of around 50%," said Rob Wainwright, international director of the Serious Organised Crime Agency.

"But we're now finding a lot that is of far lower quality -- some barely 10% pure."

"The danger is that drug dealers are cutting cocaine with other, often quite dangerous, chemicals to make it go further."

One major issue is that it is impossible for users to determine the purity of the drug by sight.

"The cocaine is a white powder and the drugs it's being mixed with are also white powder," said Dean Ames, lead drug scientist at the Forensic Science Service.

"It's impossible for someone buying a wrap of cocaine on the street to tell, just from looking at it, what it's been cut with."

The Home Office minister responsible for drug policy was bullish about the success of current law enforcement efforts.

"Along with education and treatment, enforcement has its role to play," said Vernon Coaker.

"Class A drug seizures have increased by almost a third."

"In 2005 enforcement agencies stopped 3.8 tons of cocaine, two tons of heroin, close to three million doses of ecstasy and over a million doses of LSD from harming people."