Cocaine's street price is falling as it is being cut with carcinogenic painkiller phenacetin, police say.
Cocaine is diluted with chemicals that cause cancer
The warning from the UK's Serious and Organised Crime Agency (Soca) comes as new figures are released detailing the use of Class A drugs in the UK.
Drugs like cocaine are now responsible for a £15bn annual bill in social and economic costs, says the Home Office.
And separate figures suggest Britons consume more cocaine than almost any other European Union nation.
Soca, which was specifically set up to find new ways of targeting major international criminal networks, has warned that although the price of cocaine is rising globally, its final street value is dropping in the UK.
The reason for the fall is because drugs gangs are using cutting agents - substances added to cocaine - to dilute its purity.
Customs officers are finding dogs with cocaine sewn into their bodies
The average purity of manufactured cocaine is 70%. But investigators believe dealers in Britain are now selling the drug with purity as low as 30%.
Analysis of drugs seizures has found increasing use of low cost chemical additives to bulk the weight of the drug. Phenacetin is one of the key chemicals now being used because it closely resembles pure cocaine.
The painkiller was initially banned from general use in 1968 after it was linked to bladder and kidney cancer.
The ban was later revoked - but its legal use is highly restricted because of the dangers it poses.
Bill Hughes, director general of Soca, said while cocaine was now being considered by some users as a "champagne" drug, the dangers were clearer than ever.
Demand for cocaine has risen so much in recent years in the UK that smugglers are attempting to smuggle the substance into the UK in dogs as well as human "drug mules".
HOW COCAINE'S VALUE RISES
To Europe: £18,000/kg
To UK: £28,000/kg
At 50% pure: £43,000/kg
At 30% pure: £66,500/kg
Buyers at all levels of market
In the case of dogs, the drug is wrapped in latex and then sewn between the skin and stomach lining of the animal.
Officers have found six dogs alive with 49 latex packages inside their bodies, weighing a total of 4.3kg.
Human drug swallowers will normally be expected to smuggle in up to 1kg of the drug in sealed packages.
In separate figures, research suggests that more cocaine is now being taken in the UK than in almost every other EU country.
Almost 5% of young people in England and Wales have tried the drug in the last year, while use among all adults has almost tripled in the past decade.
Use is up more than four-fold for 15 to 24-year-olds in the last 12 years.
Meanwhile, the Home Office has said that each "problematic" heroin or crack cocaine user cost the country more than £44,000 in theft, fraud, healthcare, police and judicial costs.
It estimates there are 327,000 of these problem users in England.