Smith said the decision came as she was worried about the effects of particularly strong types of "skunk" cannabis, which are found in 80 percent of drug busts on the street, The Daily Telegraph reported Wednesday.

Concerns about "binge smoking" also prompted the decision, Smith said.

"Where there is a clear and serious risk to mental health I err on the side of caution," Smith said.

It is reported Smith agreed with all the suggestions given by the Government's Advisory Council on the Misuse of Drugs, except its recommendation that the drug be left as a Class C substance.

Smith said she is aiming to get a message across to police that marijuana should be seen as a more serious problem.