At his monthly Downing Street news conference, the Prime Minister indicated his strong support for re-classifying cannabis again as a Class B drug.

His comments came ahead of the expected review this month of the Advisory Council on Drugs of the decision to downgrade it to a Class C drug.

While Mr Brown said that he would consider the council's report, he made clear that he believed that change was necessary.

"I believe that if we are sending out a signal particularly to teenagers, and particularly those at the most vulnerable age, young teenagers, that we in any way find cannabis acceptable, given all that we now know about the changes in the way cannabis is being sold in this country, that is not the right thing to do," he said.

"My personal view has been pretty well known for some time. Given the changing nature of the stock of cannabis that is coming into the country and greater damage that that appears to be doing to people who use it, there is a stronger case for sending out a signal that cannabis is not only illegal but it is unacceptable."