The former government drugs adviser forced to resign over his views on cannabis has repeated his calls for the drug to be decriminalised.
Professor David Nutt said a decision to make magic mushrooms Class A was "almost the final nail in the coffin of the rationality of the Misuse of Drugs Act" and there needs to be more "sensible, rational" regulation of drugs.
He said Government decisions on drugs were often motivated by politics rather than science, and stood by previous, controversial comments in which he declared taking ecstasy was no more risky than horse-riding.
Prof Nutt resigned as the chairman of the Advisory Council on Misuse of Drugs (ACMD) in November 2009 over the decision to reclassify cannabis from a Class C to a Class B drug.
Now chairman of the Independent Scientific Committee on Drugs, he told the Home Affairs Select Committee: "I think people have a very exaggerated perception of the harms of drugs and they tend to minimise the harms of other activities which particularly young people engage in which are potentially as harmful or more harmful."
Prof Nutt called for a system similar to the Bank of England model, whereby the ACMD could have some statutory powers "to stop party politics contaminating sensible decision-making".
"The reason the Misuse of Drugs Act was set up in the first place was to stop people playing politics with drugs," he said.
But he said in his 10 years on the ACMD, politicians would "only support recommendations which made drugs more illegal or increased the sanctions" and said in that time, cannabis was the only drug reduced in classification.
"Moving magic mushrooms to Class A was almost the final nail in the coffin of the rationality of the Misuse of Drugs Act," he said.
"Politics determined decision-making much more than science. It's easy to score political points around drugs and that's why we have ratcheted up sanctions, classes, over the last 40 years, and people have not had the courage to say, 'no, it's wrong'."