By Pallab Ghosh
Science correspondent, BBC News
The designation of drugs in classes A, B and C should be replaced with one more closely reflecting the harm they cause, a committee of MPs has said.
Cannabis is currently graded as a class C substance
The Science Select Committee said the present system was based on historical assumptions, not scientific assessment.
BBC News has seen details of a system devised by government advisers which was considered by former Home Secretary Charles Clarke but is now on hold.
It rates some illegal drugs as less harmful than alcohol and tobacco.
The new system was based on the first scientific assessment of 20 legal and illegal stimulants used in contemporary Britain.
Alcohol was rated the fifth most harmful drug, ahead of some current class A drugs, while tobacco was listed as ninth. Cannabis, currently rated a class C drug, was below both those legal stimulants at 11th.
The MPs said including alcohol and tobacco in the classification would give the public "a better sense of the relative harms involved".
They also denounced the Advisory Council on the Misuse of Drugs - which provides scientific guidance to the government - for "dereliction of duty" in failing to alert ministers of "serious flaws" in the rating system.
Phil Willis, who chairs the committee, said the current classifications were "riddled with anomalies" and "clearly not fit for purpose".
Controlled drugs are currently put into alphabetical categories, reflecting the level of penalties offences such as possession and dealing can attract.
Class A, which is the highest category, contains substances such as heroin, cocaine, ecstasy and magic mushrooms.
Class B includes speed and barbiturates. Cannabis and some tranquilisers are graded as class C substances.
Mr Willis said the only way to get "an accurate and up to date classification system" was to "remove the link with penalties and just focus on harm", adding that this meant social consequences as well as harm to the user.
He went on: "It's time to bring in a more systematic and scientific approach to drug classification - how can we get the message across to young people if what we are saying is not based on evidence?"
Speaking on BBC Radio 4's Today programme, he said: "In 1971 when the classification system was launched, that was right for the time.
"What we've had is a huge societal change over that period and what we've seen is that putting a drug into Class A does not stop people using it at all."
The alternative system was prepared by Professor David Nutt, a senior member of the Committee that advises the government on drug classification, and Professor Colin Blakemore - chief Executive of the Medical Research Council.
There are three class A drugs in the top five of the system, as well as one Class B and alcohol.
Tobacco is listed as the ninth most harmful drug and cannabis, a class C drug, comes in at number 11.
Perhaps most surprising is the presence of two Class A drugs - ecstasy and LSD - in the bottom six.
This places them well below tobacco and alcohol and a number of class B and C drugs.
Professor Blakemore told BBC News alcohol and tobacco were included in the ranking to give a "calibration of what these levels of harm mean".
He added: "That's not to say there's any argument that alcohol should be banned but it does give one a feel for the relative harm".
MOST HARMFUL DRUGS
Benzodiazepines: Wide-ranging class of prescription tranquilisers
Buprenorphine: Opioid drug used in treatment of opiate addiction
4-MTA: Amphetamine derivative sold as 'flatliners' and ecstasy
Methylphenidate: Amphetamine-like drug used to treat ADHD
Alkyl nitrites: Stimulant often called amyl nitrites or 'poppers'