Drugs policy has become a 'political football', threatening public confidence in politicians, a former government adviser on drugs warned today.

Roger Howard, chief executive of the independent UK Drugs Policy Commission, is calling for a major overhaul of drug classifications that could see ecstasy downgraded. He said it was time to take decisions about how illegal substances were classified out of the hands of ministers and base them on science, rather than political and public opinion.

The former Home Secretary Charles Clarke is understood to support such a shake-up, while Professor Sir Michael Rawlins - outgoing chair of the Advisory Council on the Misuse of Drugs, the Home Office's advisory body - is also said to be concerned about the way political and media pressure has clouded the debate over cannabis.

Rawlins will deliver a report to ministers tomorrow, which is expected to defy Gordon Brown by dismissing the Prime Minister's calls to reclassify cannabis as a Class B drug with tougher penalties. It is the third time in five years the advisory council has been asked to review cannabis and the third time it has concluded that the risk to mental health is unproven.

The Home Office said the cannabis review, and the government's response, would not be released until after Thursday's local elections.