Home Secretary Alan Johnson has stood by his decision to sack the Bristol University professor at the centre of a drugs row.

Called to the Commons despatch box to explain why he ordered David Nutt, who was the Government's top drugs adviser, to quit on Friday, the Cabinet Minister told MPs yesterday he had lost confidence in the academic.

Professor Nutt was accused of effectively campaigning against Government policy by calling for cannabis to become a class C drug and criticising the Prime Minister and Government he was employed to advise. It was the second time Professor Nutt, who heads the university's Psychopharmacology Unit at Bristol University, had embarrassed the Government.

But Mr Johnson's decision to sack him caused fury among his colleagues, with two quitting in protest and others threatening to follow. Mr Johnson said his predecessor Jacqui Smith wrote to Professor Nutt in February "expressing her dissatisfaction" with his comments on ecstasy, making it clear that his conduct was unacceptable and she did not expect it to happen again.

"It has happened again," said Mr Johnson. "Professor Nutt, much to my regret – he is a man who I respect, he is very learned in his field – but he published a paper without any notification to my department at all, contrary to the code of practice that he was appointed under. That was a re-run of February and I lost confidence."