Home Secretary Alan Johnson says Prof David Nutt went against a "long established" government principle by telling the truth.
Two members of the Advisory Council on the Misuse of Drugs have resigned in protest at the sacking.
Prof Nutt has said he doubts any "true" scientist could work for Mr Johnson.
He was sacked last Friday shortly after using a lecture to say that cannabis was less harmful than alcohol and tobacco, and saying it was upgraded to Class B - against the council's advice - for political reasons.
Mr Johnson says Prof Nutt was asked to resign "because he cannot be both a government adviser and a campaigner for truth and justice, as it goes against government policy".
He said the professor's widely quoted comment about horse riding being more dangerous than ecstasy was "stating the obvious rather than a scientific point".
DSS Comment:
A top scientific expert in their field giving information that disagrees with the government’s official stance?
Who then gets sacked as a result?
Who would have thought it?
I was expecting Prof Nutt to be found in a car park in his slippers and a dressing gown having committed suicide by shooting himself in the back with a shot gun from no less than 12 feet away.
Sacking seems quite lenient judging by many of the world's 'democratic' governments of the past 50 years or so!
