The home secretary says Prof Nutt was wrong on cannabis dangers
Home Secretary Alan Johnson said he thought his ex-chief drugs adviser was "wrong" on cannabis - but sacked him for "crossing a line" into politics.
Mr Johnson also denied Professor David Nutt's claim he had been sacked on Prime Minister Gordon Brown's order.
He told Sky News: "You cannot have a chief adviser... campaigning against government decisions."
Prof Nutt says cannabis is less harmful than alcohol or nicotine and had been reclassified for political reasons.
He told the BBC last week the government had ignored advice and upgraded cannabis to a Class B drug against the scientific evidence and "on the whim of the prime minister".
Since being sacked via e-mail by Mr Johnson, Prof Nutt has predicted there would be further resignations from the Advisory Council on the Misuse of Drugs which he headed.
Mr Johnson said he hoped there would not be resignations, adding: "This was not about Prof Nutt's views, which I respect though I don't agree with them."
He told Sunday Live with Adam Boulton: "The advice we get from scientists, whether we like it or not, is the advice.
"We have to take the decisions and be big enough and strong enough and bold enough to stand up on those decisions.
"What you cannot have is a chief adviser at the same time stepping into the political field and campaigning against government decisions. You can do one or the other. You can't do both."
Mr Johnson said that Prof Nutt had "crossed the line between offering advice and then campaigning against the government on political decisions".
He said that in the past advice from chief medical officer Sir Liam Donaldson and ex-chief scientific officer Sir David King had not been acted on, but they had not then gone on to publicly campaign against the government's decision.
Earlier Sir Liam Donaldson told BBC One's Andrew Marr Show rows between advisers and politicians were best kept behind closed doors: "I think to find yourself in a situation like this is very controversial.
"These things are best sorted out behind the scenes so that the government and their advisers can go to the public with a united front."