Drinkers in Scottish bars don't offer cash for review copies - except when Irvine Welsh is on the cover. Brian Morton kept the book, and hails a masterpiece
It calls, as ever, for something cold and clean to hold back the bile. So find a bar somewhere in the Central Belt and settle into a corner. Irvine Welsh is one of the few writers who can be read without self-consciousness in a Scottish pub, or he might be if it weren't for the instant, iconic visibility of those covers - The Acid House, the unforgettable jacket for Ecstasy, and now Filth (Cape, pounds 9.99), which comes in a choice of covers, like a pop single.

It might also be possible if Welsh had not become the spokesman and carefully defended property of a ... what? Generation? Class? Underclass? Or almost any of these up to and not including the still weaselly "nation", a concept which Welsh's fiction alone is sufficient to refute.

Here is what happens. A guy (and it is almost always a guy) clocks the cover from the bar, inch-high black and white lettering for title and author. He comes over. "Irvine's new book. Amazing. Didn't know it was out. Any chance of getting it off you?" And then comes either of two hooks into this astonishing phenomenon. The first is the most common. The second contains a valuable truth.

Mostly, it's like this: "See me. See Irvine. We used to be like that ... scarfing Eckies ... mushies ... long nights, bad fights ..."Generic stuff, but a tribute to the sense of live-as-lived.

Except that now and again you get the same lead line, and then an affectionate shake of the head. "Irvine hard? Not him. I knew him when he worked for the DSS. Home to bed by 10, sober as your granny, cuddling his clipboard." Sold or sceptical, the sense of ownership is palpable. It is surely a remarkable cultural moment when a reviewer is offered cash in a bar for an advance copy of a literary novel.

Five years ago, I had the Batemanish experience of giving Trainspotting a sour and sceptical review one nanosecond before it went mega. My objection then - and now - is that it wasn't extreme enough, that there was embedded in it a cautious, worthy clipboard-carrying voice anxious to apply a normative gloss to the antics of characters who have gone into the culture, and not just into the sub-culture, as securely as Huck Finn and Holden Caulfield.

In Trainspotting, Welsh still wrote as if driving the train with one hand on the throttle and another twitching on the dead man's handle of worthy common sense. In subsequent books, notably Marabou Stork Nightmares and, with renewed bad faith, in Ecstasy, he showed some recognition of this himself, but without resolving the contradiction. In his new book, he has.

To stumble over conclusions first, Filth is a minor masterpiece. To be sure, it is an ugly, debilitating book, but it is squarely in the classic line of classic Scottish writing, which has as often been ugly and debilitating as it has been affirmative.

Welsh has solved the problem by framing his story within the profoundly ambivalent system that is the police force. At first glance Detective Sergeant - but soon to be Inspector - Bruce Robertson is a generic bent cop, violent, racist, pocketing antiques at a crime scene, covering his own corruption with the thinnest veneer of rhetorical rightmindedness. He is a Justified Sinner for the late 1990s, and he is also R L Stevenson's Markheim, a no less playfully imagined product of Calvinism.

The malady without is matched by malady within. Whorehound and serial adulterer (his estranged partner appears in brief, unbearably touching monologues), Robertson is afflicted with a spreading eczema around his genitals. More sinister, though, he is also assailed by voices, and by the single, steadily louder voice of a tapeworm, which is eating at him from within and eating - literally, graphically - into Welsh's text. This blunt, quiet- spoken parasite gnaws away at Robertson's carefully defended rationalisations.

What is fundamentally different from Trainspotting is that the reasoning, normalising voice is not Welsh's but is located in the character of the hapless Blades. He is a Lodge brother but one who has only learned the language of Freemasonry parrot-fashion, unable to inflect its deeper rituals, unable to profit from its relentless self-promotion. Blades is a man who, rather than being eaten by his own contradictions, has ingested them whole and washed them down with several abject pints of political correctness and management-speak.

Much of Filth is off-the-peg Welsh. Anal sex with Amsterdam hookers, drugs and drink, obscene calls to Blades's wife, casual violence protracted past all logic. The difference is that sex increasingly seems to represent not a defiant trade with, but a way of violently expunging Original Sin. Drink, as so often in Scottish culture, is a heroic remedy, here an unsuccessful specific against worms.

The most profound addiction, though, is to language itself, as a means of bombarding the material world, irradiating it with the meaning it lacks. That was true of Trainspotting, though it retained a realist core; it was true of Marabou Stork Nightmares as well, though there language took on a new and more radical significance. In Filth, language contains the seeds of its own destruction; it is literally the enemy within.

The worm acquires its language as Robertson loses his own ability to articulate, to act other than galvanically. Its first manifestation is as nothing but rows of noughts and a single urge - "ooooeatoooooooo" - surrounded by ripples of peristalsis, but steadily and threateningly it acquires a cool, diabolical sophistication of discourse as "Host" and "Other" exchange roles.

Filth is about the process of story-telling itself. The stories are of markedly different kinds. There is Robertson's venial bluster; there is the voice of the worm, which destroys one kind of meaning to replace it with another; and there are the bland evidential narratives that make up much of police work.

For sheer concentration of language, there has been nothing like this since Hubert Selby Jr's The Room and the Demon. The danger that lies in wait for Welsh is exactly the one that eventually ambushed Selby in the elegiac Requiem for a Dream, with its offstage, offpage nod to "faith in a loving God". Welsh has often threatened to fall silent, to devote his energies to other aspects of the cultural nexus.

We have been slow to hold Andrew Lloyd Webber to his promise of leaving Britain forever after the events of 1 May 1997. It would be a more generous act to hold Welsh to his less dogmatic, but probably more sincere undertaking. To repeat, Filth is a masterpiece. It closes with a last, apocalyptic evacuation and a diminishing row of zeroes. The rest is, and ought to be, an appalled silence.