The Tories say the experiment in the decriminalisation of cannabis has allowed drug gangs to flourish.
The claim comes from shadow home secretary Oliver Letwin.
People with cannabis for personal use in Lambeth have been let off with confiscation and a warning to allow police officers to focus on the war on hard drugs.
Scotland Yard Deputy Commissioner Ian Blair, the force's second-in-command, hailed the scheme an undoubted success just days ago.
And Home Secretary David Blunkett is expected to announce the effective decriminalisation of the drug across the country within weeks.
But Mr Letwin insisted on the BBC1 programme Breakfast with Frost the pilot project in Lambeth, south London, has only helped the drug gangs tighten their grip.
"We have been seeing in Lambeth an appalling example of the gangs getting back in charge," the shadow home secretary said.
"The medical evidence suggests that cannabis and cannabis derivatives are pretty dangerous to the mind," said Mr Letwin, who has admitted accidentally smoking cannabis when friends at university spiked his pipe.
"It is pretty difficult to imagine a young person who is taking that kind of drug regularly prospering in school and getting a good job.
"And again, going back to Brixton, there seems to be every evidence of these gangs and dealers dragging people on from cannabis to other drugs."