Iain Duncan Smith has challenged the prime minister to say whether he still supports the pilot cannabis scheme in south London.

His intervention came amid police criticism of the Brixton scheme - which effectively decriminalises the possession of cannabis for personal use.

Blair said it was too early to tell whether the pilot project was a success.

He said it was important that "we consider this on the basis of all the evidence" before deciding whether the experiment should be scrapped or extended.

Iain Duncan Smith said that drug dealing levels were soaring in Lambeth and called for the project to be scrapped.

"Community leaders are all complaining about it. One has actually said that the police have abandoned the streets to the dealers," he said.

"A few years ago the prime minister promised to make Britain the hardest and toughest place to be a drug dealer in the western world. Will he stop the scheme now?"

Blair dismissed suggestions that local people were unanimously against the pilot drugs project and accused the Tory leader of pursuing a "pointless line of questioning".

"It is surely important that we do learn the lessons. There are differences of opinion about whether it has worked or not. I can assure him that if we come to the view, if sensible people come to the view, that it is not working then of course we shall not do it," he said.

"We will take into account the views of community leaders, the views of the police, the views of experts on the ground."

Blair later hinted that the government was set to back a pilot project on the medicinal use of cannabis.

Liberal Democrat backbencher Alistair Carmichael urged the prime minister to step in on behalf of a constituent with multiple sclerosis who had been arrested for possessing cannabis.

"We are reviewing this very issue now between cannabis and people with diseases such as multiple sclerosis," said Blair. "We do understand there is a distinction between those who need it for medical purposes and those who don't."

The prime minister said he sympathised with the woman concerned - although he stressed it was an issue for the authorities to decide.