Duncan Smith has a bad trip on his day out to the streets of Brixton

The trouble with buying drugs from strangers in places you would not normally be seen dead in is that you have no idea what kind of rubbish is in them or what kind of harm they can do to you.

It was a lesson apparently not learnt by Iain Duncan Smith yesterday when he swallowed everything he was offered during a bizarre meeting that could influence his party's future drug policy.

Conservative Central Office announced that its leader was to meet residents of Brixton to highlight the "failure" of the Lambeth experiment in relaxing the police's approach to the use of cannabis. However, instead of marching along the mean streets of Brixton Market and Coldharbour Lane, Mr Duncan Smith met eight grandmothers, two members of the clergy and a youth worker in a church.

The result was something like a bad trip as the Conservative leader emerged from the meeting to denounce the Lambeth experiment and to warn that Home Office plans to extend it would end in disaster.

His comments were timed to coincide with an announcement expected today from David Blunkett that cannabis is to be reclassified from a class B to a class C drug, meaning that possession of small amounts would no longer be an arrestable offence.

The Conservatives' drug summit took place at the Brixton Baptist Church, where the minister, the Rev Chris André-Watson, and his flock have some genuine concerns over what they see as a growth in drug dealing in the area since the policy of relaxation was introduced by Commander Brian Paddick, the controversial officer who has subsequently been put on desk duties at Scotland Yard.

Since the scheme was introduced, under which users are cautioned instead of prosecuted, they feel drug dealers have become bolder, cannabis smoking has gone public and more young people are being drawn into it. "We have grave concerns over the way the policy has been implemented," said Mr André-Watson. "These people are now selling cannabis openly. There is concern among our mothers and grandmothers that their children are being sucked into a criminal culture."

Others expressed similar concerns, although two out of four interviewed by The Independent said they wanted to see the results of the experiment before calling for it to be scrapped. Two out of the four did not know who Mr Duncan Smith was.

Outside, Shane Collins, the Green Party's spokesman on drugs and a member of the police's consultative group on drugs, was miffed that, as a local resident, he had not been invited. "There are problems with cannabis dealers selling crack and heroin, but that does not mean you should treat cannabis in the same way as the hard drugs," he said. "If we took cannabis sales into cafés, you could raise money that could be put back into the community, you could separate cannabis from the class A drugs and arrest the hard drug dealers left out on the streets."

Since the experiment was introduced, 1,400 hours of police time have been freed and the number of class A drug dealers arrested is up by 10 per cent. A recent Mori poll showed that 83 per cent of the residents of Lambeth thought the experiment was a success.