Scientists are calling on the UK government to agree a new statement of principles to protect the independence of its scientific advisers after the home secretary, Alan Johnson, sacked David Nutt as his chief adviser on drug misuse.

Mr Johnson told parliament that he had “lost confidence” in Professor Nutt, a leading expert on the relative risk of different drugs, after he publicly criticised the government’s decision to reclassify cannabis from class C to the more serious class B.

The sacking, and Mr Johnson’s assertion that Professor Nutt was “campaigning” for a change in government policy, caused a media furore, and the home secretary was forced to explain his actions to …