In a scathing report entitled Drug Classification: Making a Hash of It?, the Commons science and technology committee says there is no consistency in the way drugs are classified A, B or C and no evidence to support the official view that the classification has a deterrent effect.
The MPs are highly critical of the Advisory Council on the Misuse of Drugs, the government's key advisory body on drugs policy, calling its failure to alert the home secretary to the serious flaws in the system "a dereliction of its duty".
Phil Willis, the committee chairman, said: "The Home Office and ACMD approach to classification seems to have been based on ad hockery and conservatism.
"It is obvious that there is an urgent need for a root- and-branch review of the classification, as promised by the previous home secretary [Charles Clarke]," Mr Willis said.
"We all know that the current home secretary [John Reid] has other things on his mind but that is not an excuse for trying to kick this issue into the long grass."
The MPs condemn the government's "proclivity for using the classification system as a means of 'sending out signals' to potential users and society at large - it is at odds with the stated objective of classifying drugs on the basis of harm." Individual drug classification reviews are "launched as knee-jerk responses to media storms", they say.
The committee proposes a new scale, decoupled from criminal penalties and based on the best scientific evidence of the harm done by each drug - not just to users but to society as a whole.
It should also give the public a better sense of the relative harm of alcohol and tobacco.
On individual drugs, the report says the government contravened the spirit of the Misuse of Drugs Act when it put "magic mushrooms" into Class A without consulting the ACMD. But the committee undermined its own credibility by not speaking out on the issue.
The MPs express surprise and disappointment that the ACMD has never reviewed the evidence for ecstasy's Class A status. And they say the committee's recommendations on the status of methylamphetamine ap-peared to rely on politics and outside pressure rather than science.
Mr Willis, a Liberal Democrat, said the conclusion was that "it is 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 the evidence?" he asked.