The government will reclassify cannabis from a class C to class B drug today.
The change in the law will come into effect next year.
The move directly contradicts the government's own expert advice on the matter.
Members of the Advisory Council on the Misuse of Drugs overwhelmingly concluded in their last report that cannabis should remain a class C drug.
Liberal Democrat spokesman Chris Huhne MP criticised the decision.
"What is the point of expensively assembling a panel of experts if ministers are not going to listen to them?" he asked.
"Cannabis usage has actually fallen since the reclassification, which makes this a spectacular example of a misguided and expensive solution to a diminishing problem."
Mental health charity Rethink have spoken out against the reclassification, saying it will not effect the number of people using cannabis.
Gordon Brown is the second prime minister to over-rule the council since it was created in 1971.
Just under 18 per cent of 16-24 year olds have used cannabis in the last year, compared to 20.9 per cent in 2006-07.