Fifty years ago, Lenny Bruce, the American comedian who was pursued relentlessly by the police for his drug use, remarked that cannabis would be legal soon, "because the many law students who now smoke pot will some day become congressmen and legalise it in order to protect themselves". Since then we have had at least two US presidents and countless congressmen who have used drugs, but changes in the punitive US drugs laws seem as remote as ever.

In Britain, many cabinet and shadow cabinet members have admitted to using cannabis but, rather than relaxing the laws concerning the drug, they are planning to tighten them. Today the Advisory Council on the Misuse of Drugs is due to hear evidence on whether or not cannabis should be reclassified from class C up to class B. The council considered this issue in 2005 and concluded then that "although cannabis is unquestionably harmful, its harmfulness does not equate to that of other Class B substances either at the level of the individual or of society".

This time the hearings are pointless. Gordon Brown and the home secretary, Jacqui Smith, have already indicated that they are minded to reclassify the drug upwards, whatever the council has to say. Brown has said that "drugs are never going to be decriminalised". The received wisdom, inside the cabinet and among much of the media, is that it was an error on the part of the then home secretary, David Blunkett, to reclassify cannabis down from B to C in 2004, because it "sent the wrong message". And the increased strength of hydroponically grown skunk is cited as one reason for the change. The sunny climate in which Rosie Boycott launched a legalise cannabis campaign in the Independent on Sunday in 1997 has clouded over. The IoS itself has recanted and issued an apology.

There is no dispute that cannabis can cause significant harm. Teenagers, heavy users and those with a predisposition to mental health problems are at risk. No one denies that. Transform, one of the most rational of the organisations monitoring UK drug laws, will be submitting evidence to the advisory council, saying that "the fact that [cannabis] is produced and supplied via a profit-driven underground criminal market has been the driver for the increasing prevalence of more potent strains, which deliver increased profit-to-weight ratios".

Some senior former police officers, like Tom Lloyd, former chief constable of Cambridge, have also argued for a change in the laws. "This is about taking the control of drugs in this country out of the hands of criminals and into the hands of responsible authorities," Lloyd has said. Many still in the police privately agree.

But it would take a brave politician to suggest a sober debate on cannabis, let alone the whole basis of the drug laws. The Lib Dems and the Green party still favour that debate. The former's policy is to seek "to put the supply of cannabis on a legal, regulated basis, subject to securing necessary renegotiation of the UN conventions". It opposes the government's decision to reclassify regardless of what the ACMD has to say.

But what of the two main parties? Shadow cabinet member Alan Duncan wrote in the book Saturn's Children that "logic suggests that the only completely effective way to ameliorate the problem, and especially the crime which results from it, is to bring the industry into the open by legalising the distribution and consumption of all dangerous drugs, or at the very least decriminalising their consumption". In 2002, the home affairs committee examining drugs policy recommended that "the government initiates a discussion within the Commission on Narcotic Drugs of alternative ways - including the possibility of legalisation and regulation - to tackle the global drugs dilemma". David Cameron was a member of that committee. But this is not Conservative policy now, nor will the party dare to offer it for debate for fear of being called soft on drugs. It now backs the government on reclassification.

It is time for politicians to take a deep breath and say in public what many say in private: that the drug laws are not working, that the illegal trade is responsible for much of our most corrosive crime, and that it is time to have a debate nationally and internationally about addressing the catastrophic effects of prohibition. Reclassifying cannabis upwards is a grandstand gesture with no relevance to those whose lives are damaged by drugs or by the drug laws that compound and exacerbate that damage. The country does face an urgent addiction problem. But the name of our addiction problem is alcohol. If the government wants to send messages, the first message should be in a bottle.

The real "softies" when it comes to drugs are the politicians who refuse to engage in debate for fear of being called soft on drugs. So now, instead of that debate, we appear to be heading towards Reefer Madness II.

duncan.campbell@theguardian.com