Blair announced a crackdown on middle-class drug users last week. His latest comments reveal how he hard is prepared to push the initiative. Senior officers say that London and other big cities are in the thrall of a drugs “epidemic”.

Blair, who took over from Sir John Stevens as London’s police chief last week, said: “We are not going to burst through doors to raid Islington dinner parties, but I do want to make people concerned that they might be buying their drugs from a police officer: that would be an interesting idea, wouldn’t it?”

In a warning to the estimated 250,000 people who are believed to use the drug as a “recreation” each weekend, Blair said: “People seem to think the price of a wrap of cocaine is 50 quid, but the cost is misery on estates here and a trail of blood back to Colombia.

“Someone has died to bring it to a dinner party. People who wouldn’t dream of having a non-organic vegetable don’t seem to notice the blood on their fingers.”

Announcing his crackdown last week Blair spoke of making “a few examples of some people” to reinforce the point that nobody is above the law.

“There is a sense that people think that in certain fashionable clubs, restaurants and dinner parties it is okay to do drugs,” he said. “All I can say is that people may find out that it is not.”

Blair distinguishes between cocaine, a class A drug, possession of which carries a maximum seven-year prison sentence and fine, and cannabis, which was reclassified from a class B to class C drug last year with a maximum sentence for possession cut from five to two years. He has said he is “relaxed” about cannabis and does not consider arresting cannabis smokers to be the best use of police resources.

A senior Scotland Yard officer said yesterday that Blair’s intention to target middle-class users signals a change in police tactics.

At present a joint Scotland Yard and Customs unit code-named Middle Market is tackling street dealers. Undercover detectives posing as potential customers approach the dealers to conduct “test purchases”.

Under Blair’s policy, undercover officers would pose as local dealers offering to sell cocaine and other drugs. They would operate in areas such as Chelsea, Kensington and Islington, where drug use among the middle classes is thought to be widespread.

Nightclubs and pubs in the West End, where customers retire to toilets to snort cocaine, are expected to be raided over the next few months.

Colombian cocaine cartels have increasingly focused on the British and European markets as it has become more difficult to smuggle huge quantities into America.

There has been a four-fold increase in seizures of cocaine in the past two years. The price of a gram has fallen from £60 a decade ago to as little as £40 in some areas. The price of ecstasy, heroin and cannabis has also dropped.

Blair also favours the use of phone-tap evidence in trials of terror suspects, a position that puts him at odds with Charles Clarke, the home secretary, who has recently ruled it out.

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