The controversial pilot scheme that relaxes Scotland Yard's attitude towards cannabis possession has been criticised by the head of the US drug enforcement administration for increasing drug usage.

Asa Hutchinson, director of the DEA, said that the scheme in Lambeth, south London, had led to a rise in cannabis users and led children to believe it was legal.

The pilot programme, which has been running since July last year, means people caught with cannabis are given on the spot warnings, rather than being cautioned, arrested and possibly charged. An interim study carried out during the first six months of the scheme indicated it had been a success and led to it being extended. This is seen as a sign that the commissioner, Sir John Stevens, is keen for it to roll out across the capital.

The idea is to allow officers to tackle more serious crimes and target drug dealers, rather than spending time dealing with people caught with small amounts of cannabis.

Mr Hutchinson, who is attending a crime conference in London, said: "Clearly, the evidence is that the pilot project in that area where they only issued citations for marijuana use, rather than an arrest, is that it increased usage.

"Wherever you are dealing with harmful drugs, that is not the objective that we want to have.

"As I travelled the neighbourhood, I asked about the young people and the impact on them and the response that came back was that most of them think it is legal now."

A Metropolitan police spokesman last night said: "The pilot scheme continues and its results will be evaluated. We welcome contributions to this debate."