People caught possessing cannabis for a second time will be given an £80 fine instead of a warning, the Home Office has confirmed.
The move comes as the government launched a robust defence of its decision to reclassify the drug back to class B.
"While cannabis has always been illegal, reclassifying it to a class B drug reinforces our message to everyone that it is harmful and should not be taken," said home secretary Jacqui Smith.
"Fewer people are taking cannabis, but it is crucial that this trend continues."
That claim was rubbished by drug reform groups who point to advice from the government's own scientific advisory council calling for the drug to be left at class C.
"Cannabis use was falling when the government dismissed the advice of its appointed experts and decided to reclassify cannabis to class B and use has continued to fall," Danny Kushlick, Transform's head of policy, told politics.co.uk
"It's latest announcement to increase penalties is yet more populist posturing that flies in the face of the science and evidence that harsher penalties are no deterrent to users."
Cannabis possession was decriminalised by former home secretary David Blunkett in 2004, making it a largely non-arrestable offence.
The reclassification of cannabis will take place on January 26th 2009.