"Unless and until they show leadership on the issue, the drugs trade will remain in the hands of organised criminals and unregulated dealers."
The Liberal Democrats commissioned a study - conducted by a panel of scientists, academics and police chiefs - that suggested the drug should be available in specialist shops to over-18s.
Mr Lamb, the former Lib Dem health minister, has proposed a Bill to legalise cannabis.
In March he said: "A regulated market in the UK will take profits out of the hands of organised crime and reduce both health and social harms."