"Prof Murray said: "Individuals who - perhaps with some mild predisposition - would not otherwise have developed schizophrenia will do so because of taking cannabis. It's a bit like how people with only a minimal predisposition to diabetes will develop it if they eat too much."
Dr Iddon, the chairman of the all-party parliamentary group on drugs misuse, said the study did not convince him it was time to return cannabis to class B.
"I don't think the causal link has been proved. I think cannabis might - possibly for genetic reasons - trigger psychosis at an earlier age."
The MP, who is also a member of the science and technology select committee, said there was a danger of criminalising "hundreds of thousands of young people" if the status of the drug was changed.
If Gordon Brown changes the class of the drug, it won't be evidence-based but for political reasons," he said.
"Since we reduced the classification of cannabis from B to C the usage is going down, so what's the point of muddying the debate again by this yo-yo political policy?"