Tony Blair, Jack Straw and Sir Paul Condon, Britain's top policeman, are almost certainly holding in their wallets banknotes tainted with cocaine. And so is virtually every Londoner, according to a survey published today.

Scientists have found that more than 99% of banknotes tested from the capital were contaminated by the class A drug.

Of 500 notes tested by experts at Mass Spec Analytical, just four were clean. Nearly one in 20 were found to have high levels of cocaine suggesting they had been in close contact with the illegal drug.

The findings, commissioned by BBC Newsroom South-East, suggest that almost every banknote in London contains traces of cocaine.

The results do not suggest that most notes are used to snort cocaine; rather that most are contaminated by currency handled by dealers or users.

The notes tested were supplied by the Bank of England's returned note centre.

Scientists also found that 4% contained traces of ecstasy, with one in 100 notes testing positive for high levels of both cocaine and ecstasy.

Figures suggest that cocaine has become the fastest growing "recreational" drug among 20- to 24-year-olds. The British crime survey released last month found cocaine use in that age group had doubled over two years.

Cocaine is believed to be spread on to notes by bank sorting machines, transfer by human hands that have handled the drug, and by dirty notes rubbing against clean notes while stored.

Joe Reevy, of Mass Spec Analytical, said: "Once you've taken a snort, the compounds will be in the oils of your skin and they'll get transferred to the notes you handle. That's the main way in which the cocaine gets on to the notes.

"When you test notes that have been used directly to snort cocaine, you get a great big reading and the machine takes quite a while to settle down. You don't miss the difference."

In some parts of Britain prices for cocaine have plunged from £80 a gram to £40 a gram. The drug, traditionally associated with high earners, is increasingly popular among clubbers and even schoolchildren.

Roger Howard, of the standing conference on drug abuse, said: "Cocaine prices have been relatively stable and have even come down. Like any market where prices are stable or falling, you get rising consumption. That's an economic law. Prices are falling because of supply-led reasons, such as bumper crops in South America of coca and more land cultivating it."

But for those wanting a fix, there is little point in scraping notes in the hope of producing enough cocaine for a line or two. Some amounts found were minute, just 80 nanograms.

As cocaine becomes cheaper in real terms, its high class image is helping to make it attractive to teenagers.

The London-based Youth Awareness Project, which recruits young former drug users to educate schoolchildren, said it was aware of children as young as 14 trying the drug and carrying it in schools.

A project member said: "Money and fashion, that's the image kids have of it, and also power. Young cocaine users know if they take cocaine that's a little reputation they've established. If you start selling it and everything, the more power you will have."