The number of people found carrying cannabis has risen by 29% in London since laws were relaxed in January, a report has said.
The Met claims cannabis possession arrests have fallen
Met Police figures show 6,321 were found in possession of the drug from April to August 2003. That rose to 8,148 in the same period this year.
London Assembly Conservatives said the class C drug should be returned to class B status, to reduce its use.
But the Met said the number of actual arrests fell by 53% since downgrading.
The force says this has saved almost £425,000.
But the report adds the reclassification had sent out a mixed message to members of the public.
Also, front line officers found people with cannabis on them more confrontational because they believed they would not face arrest.
Bob Neill, leader of the London Assembly Conservative group and member of the Metropolitan Police Authority (MPA), said: "The reclassification would seem to have made it harder, rather than easier, to enforce the law.
"If cannabis were reclassified back to a class B drug, everybody would know where they stand, cannabis use would fall and police could return to arresting those who use illegal drugs."
Fellow MPA member Richard Barnes told BBC News: "Research has shown that cannabis is not the harmless little narcotic that we are led to believe it is."
The London boroughs of Lambeth, Hackney, Westminster, Merton and Barnet recorded the highest numbers of people caught carrying cannabis.
While Harrow, Hammersmith and Fulham and Barking and Dagenham had the lowest rates.