People caught with cannabis now face arrest in the area where plans to relax the drug laws were first tested.
Drugs laws were relaxed in Brixton
From Monday anyone found with even a small amount of cannabis in Brixton, south London, will be arrested.
When the drug was downgraded to class C last year the police's attitude to people with small amounts was relaxed.
The move is part of a clampdown on dealers, who avoid arrest by carrying only a small amount, and those who visit Brixton to buy drugs.
Current legislation allows arrests to be made if cannabis is deemed problematic to an area.
Community leaders have welcomed the move saying they hoped it would stop 'drug tourists' coming to the area.