For most of human history, mind-altering substances have been regarded as a source of innocent pleasure, or perhaps something better than pleasure: religious rapture, aesthetic insight, or rushes of courage or love. But they can do obvious harm as well: occasional adventures may make you clumsy, reckless, deluded or obtuse, and if they coalesce into steady habits you could end up as an outcast or a helpless invalid.

As long as such effects were confined to upper-class adults like Samuel Taylor Coleridge, Thomas de Quincey or Florence Nightingale, they could be treated as individual misfortunes; but when they were observed among the productive classes and their children, moral indifference was hard to sustain. The gin craze in 18th-century London provoked a handful of panicky government reactions, and a century later the British state embarked on systematic regulation of the opiates contained in popular patent medicines. But specific legislation against so-called narcotic drugs did not start till the first world war, when the government  worried about public order, national security and military discipline  imposed restrictions on pub opening hours, and made unauthorised possession of cocaine and opium a criminal offence.

Cannabis was added to the list of forbidden substances shortly afterwards, but it always carried an aura that set it apart. It was held to be relatively harmless  more like alcohol and tobacco than opium or cocaine  and in any case, it was associated with bohemian intellectuals in studios and jazz clubs rather than scowling proletarians in dens of vice and iniquity. If there were steep increases in cannabis consumption after the second world war, they were concentrated among the kind of people who were considered too genteel to be of any interest to the police.

What happened next is the subject of the latest polemic by the conservative columnist Peter Hitchens. The story he tells, though it has some difficulty emerging from his tangled exposition, is meant to shock and appal. In the beginning there were the Beatles, scattering sinister references to drugs in their sweet-sounding songs. Then there were the celebrity drug-busts: Donovan and his naked girlfriend in 1966, followed by Mick Jagger and his rolling friends. To Hitchens's consternation, these criminals managed to escape the obloquy they deserved. In the first place, there were gaggles of teenage fans "voiding their bladders", as he puts it, in uncontrollable excitement. Then there was the louche coterie of lascivious libertarians who placed an advertisement in the Times in 1967, calling for complete decriminalisation of cannabis. And finally there was utter calamity in the form of the cultural revolution of 1968.

Hitchens is not the kind of conservative who rhapsodises about the persistence of tradition in British life. As far as he is concerned, the institutions that once made Britain great  from parliament to the ancient seats of learning, from monarchy and the common law to the Church of England  have all been hollowed out and subverted by cynical soixante-huitards.

He traces the damage to what he calls the "central committee of the British cultural revolution", most of whose members, he claims, were signatories of the petition in the Times: David Hockney, Graham Greene, Jonathan Aitken, Alasdair MacIntyre, Kenneth Tynan, Francis Crick and David Dimbleby, among others. If these names strike you as rather safe, that only demonstrates the triumph of their silky stratagems. With the help of other university-educated subversives  from Tony Crosland, Harold Wilson and Roy Jenkins on the left to Reginald Maudling, William Rees-Mogg and Michael Portillo on the right  they worked tirelessly to turn Britain into a country with no moral aspirations apart from sex, drugs and rock'n'roll. They recruited a number of luckless ladies as well, notably Diana Runciman ("divorced"), Barbara Wootton ("battleaxe") and Joan Bakewell ("adulteress").

If you want their monument, you need only open your eyes to observe a Britain where parents are no longer allowed to be parents, where teachers will not teach, and where the police are more interested in political correctness than the prosecution of crime. Divorce, you will find, has become "easier than ending a car-leasing agreement", and restrictions on the sale and use of alcohol have been abolished. In due course, you will become aware of the most terrible scandal of all: that the law against cannabis and other stupefying narcotics is now "so feebly enforced that it might as well not exist".

If you still have difficulty recognising Hitchens's version of contemporary Britain, that only confirms the power of the Big Lie. Ever since 1968, our unelected rulers have given an impression of fighting a war against drugs, while surreptitiously dismantling our last defences against them. Without anyone realising what they were up to, they have wiped out the old ethic of hard work and honest trade, and turned us into a nation of pot-smokers.

Hitchens is a virtuoso of the magniloquent non-sequitur. He devotes many pages to denouncing research into the risks associated with different drugs ("meaningless", he calls it), and spends just as many stringing together anecdotes to suggest that cannabis is the most harmful drug of all. He notes that American attempts to prohibit alcohol in the 1920s were a self-defeating disaster, but goes on to call for exactly the same no-tolerance approach to cannabis in Britain now. And when he offers a bemused account of the government's success in reducing the use of tobacco in Britain, he manages to overlook the obvious inference: that if cannabis is as dangerous as he supposes, then it ought to be explicitly decriminalised and subjected to the same controls as tobacco.

I have never been very impressed by the idea that human nature changed in 1968. And Hitchens's unhinged affirmation that, since that date, young people have considered themselves entitled to everything they want without any effort or study is not going to make me change my mind. In a period of glorious achievement in classical music and theatre, I find it hard to believe "high culture, effort, self-discipline and patience" have lost out entirely to "young men and women who brawl and spew in the midnight streets of our great cities".

I am surprised, however, to find that someone who claims to respect culture and discipline chooses to mock Reginald Maudling as a "Hegel enthusiast", as if habitual Hegel-reading were a wretched self-indulgence rather than an arduous long-term cultural investment. And I am astonished that he can base an entire book on a few cuttings from the Evening Standard, the Daily Mail and the News of the World.

But it occurs to me that I may always have been wrong about 1968. Perhaps it really did change the world  not by turning us all into junkies, but by unleashing the virulent strain of hysterical moralising that Hitchens appears to be suffering from. Like many other members of his generation  fogeys of the left as well as the right  he seems to be addicted to a form of self-righteousness so intense that it amounts to moral racism. His own instincts strike him as so self-evident that anyone who does not share them has to be dismissed with total contempt. If we are indeed surrounded by cultural ruin, then Hitchens is a fine specimen of it: one of those manic preachers who have been shouting so long that they can no longer hear any voice apart from their own.

 Jonathan Rée's I See a Voice is published by Flamingo.