From the cod-medieval poems of Thomas Chatterton to the pseudo- Shakespearian verse of `Vortigern' and beyond, Paul Taylor argues that a fake can be as valuable a reflection of its time as any `genuine' work of art.

Its first performance had originally been scheduled for 1 April 1796 at Drury Lane. This would, indeed, have been a more appropriate date than the second of the month, to which the premiere was, superstitiously, shifted. For Vortigern - trumpeted as a long-lost, recently unearthed historical tragedy by Shakespeare - left more than a few people feeling like an April Fool.

The play had taken in luminaries such as Henry Pye, the Poet Laureate, and James Boswell, Dr Johnson's biographer, who sank to his knees before this and other documents from the same source (these range from a letter addressed to "Anna Hatherrewaye", accompanied by a lock of Bardic hair, to a few leaves of Hamblette). It had pulled in a starry cast, including John Philip Kemble, the greatest actor of the day, and Mrs Jordan, the King's mistress.

But then, with timing that can't be accused of overweening tact, on 31 March a scholarly tome was published - Edmund Malone's An Inquiry into the Authenticity of Certain Miscellaneous Papers and Legal Instruments - which entirely discredited both the documents and their forger, one William Henry Ireland, the 19-year-old son of Samuel Ireland, Bardolater extraordinaire. As pre-show publicity, this was something of a downer. Kemble had always had his doubts and, on the first and only night, he encouraged the audience in the jeering and orange-throwing that brought the proceedings to a halt half-way through Act 5. So, when Vortigern opens next Thursday in Joe Harmston's production at the Bridewell, off Fleet Street, it will - strictly speaking - be the world premiere.

Why bother to resurrect this curiosity either in the theatre or here on this page? A crazy quilt of Shakespearian motifs and echoes, it tells the story - with abject apologies to Macbeth, King Lear, Cymbeline and As You Like It along the way - of a military hero who, rewarded with half the kingdom for his victories, allows lust and ambition for total sovereignty to go to his head.

As to the qualities this work may or may not possess in its own right, the director of the new production will have his say at the end of this piece. Quite separately from that, though, Vortigern is well worth pondering for its considerable cultural significance, both as a sign of its times and as a glinting link in the false-metal chain that leads to our own post-modern society, where artistic hoaxes, quite brazen about themselves, can be passed off as the genuine article. It isn't fanciful to suggest that there's a route from the world that created Vortigern to the world that created that wunderkind de nos jours, Martin McDonagh, author of The Beauty Queen of Leenane.

Vortigern was, you might say, a pseudo-event that was waiting to happen. It marks the confluence of powerful forces that were then re-shaping the culture. A crucial change had been brought about by the Copyright Act of 1710. This introduced the concept of literature as personal property, so it's no accident that this was also the century of classic forgeries - among them, the "Marvellous Boy" Thomas Chatterton and his invented medieval poet, Rowley, and James Macpherson with his supposed discovery of the Gaelic bard, Ossian.

The rise of Bardolatry and the new romantic conception of "genius" were fresh energies calculated to produce relic-worship. When the theatres had reopened in 1642, Shakespeare's stock had been lower than that of Ben Jonson or Beaumont and Fletcher. Cut to 1769 and you have David Garrick conducting Jubilee celebrations in a Stratford full of pilgrim-tourists avid for their souvenir-equivalent of a splinter of the True Cross: a piece of the mulberry tree allegedly planted by Shakespeare's hand. A manuscript by that hand would obviously trigger transports of fetishism.

So someone was going to turn forger and William Henry Ireland had the right psychology, itself partly a product of the times. William Henry's heart belonged to Daddy, whose heart belonged to, well, Shakespeare. In his money-spinning confessions, the son pleaded that he had only faked the documents in order to please his pater. William Henry was inspired by the example of Chatterton, whom he had come across in a contemporary novel, Love and Madness. In a long digression about the poet-faker, this novel had said of the noun "forgery" that "for Chatterton's sake, the English language should add another word to its Dictionary": how could "the deception of ascribing a false antiquity of two or three centuries to compositions for which the author's name deserves to live for ever" be considered a crime?

The fact that Chatterton - the 18-year-old suicide who became for the romantic poets an icon of lonely, harried genius - was also, in one sense, a fraud was the kind of paradox not likely to be wasted on a certain Oscar Wilde. Wilde, who lectured on Chatterton and who, in the memoir "Pen, Pencil and Poison", celebrated the forger and poisoner, Thomas Griffiths Wainewright, provides us with the next move in the game. The forger, for Wilde, is not an aberrant would-be artist, but the very type of the artist. Debasing a currency that is already debased, the artist is the covert subversive - though it might be added that this does involve collusion with the culture under attack. Whatever else Andy Warhol's screen "printing" of dollar bills may be, semiotically speaking, they are not anti-capitalist.

One thing you might say in defence of forgery-fostered postmodernism is that it has saved people like the novelist Peter Ackroyd and the playwright Martin McDonagh from a life of crime. In an earlier period, they would have had to fake - er, sorry, pastiche - in private. Ackroyd's novel Chatterton piles fate on fate, imagining the discovery of papers and a painting that suggest that the Marvellous Boy lived on, in fact, to his 50th year, the suicide itself a fake and a career-move. The fear of never being able to say (or rather write) anything in your own voice - of being trapped in a literary echo chamber - haunts this book. The tentative intimation that one of its characters may one day be able to do so, to speak with his own voice, is not best supported by the fact that Ackroyd's own ear deserts him in the contemporary passages and that the book's donnee - the twist of linking the forgery of a suicide with the forgery of a work of art - certainly looks as if it's lifted from an earlier literary source: Wilde's "The Portrait of Mr W H".

Which brings us to Martin McDonagh, who is making quite a career for himself - first play premiered at the Royal Court, second play premiered at the National Theatre - on the back of the works of John Millington Synge (1871-1909). The South London-born McDonagh can produce comic pastiche "Oirish" by the Irish mile. His admirers claim that the zesty ersatzness of his style is at one with his subject matter - the idea that there is no such thing as the real Ireland, only various myths. They point out that St John Ervine once accused Synge of being "a faker of peasant speech": McDonagh, our new playboy of the imaginary western world, is simply upping the irony of it all.

So that's all right, then, is it? Not for some of us, who see this young dramatist as the casualty rather than as the champion of postmodernism. Sitting through one of his plays, you hear the roar of art-on-art feedback; you hear the increasingly mechanical laughter of the audience; you rarely hear the beating of a heart. There's an eerie lack of emotional investment in these sadistic, opportunistic plays. Even if you were completely ignorant of Synge's work, you would, I suspect, sense a hollowness - as when you tap a trompe-l'il bookcase.

McDonagh exists. I even know people who have interviewed him. But, for my peace of mind, I prefer to think of him as a hoax perpetrated by some committee of postmodernist pranksters - a theatrical equivalent of the "Bruno Hat" exhibition of 1929, which introduced London to a fake modernist painter: the pictures by Brian Howard, the catalogue notes by Evelyn Waugh, and the non-existent genius impersonated by Tom Mitford.

As for Vortigern, Joe Harmston, the director of next week's world premiere, thinks this fake is full of genuine drama. In A Question of Attribution, Alan Bennett's teasing meditation on fakes and forgeries, the fraudulent Anthony Blunt (former Surveyor of the Queen's Pictures and sometime Soviet spy) declares that any forger is "of his time" and, however slavishly he imitates, he does it in the fashion of his time, in a way that is contemporary - "and with the passage of years it is this element that dates, that begins to seem old-fashioned and which eventually unmasks him".

This, as Harmston agrees, is certainly the case with Vortigern. What the play reflects is not Shakespeare but the taste of the 18th-century adaptations of him. It doesn't end, for example, with the hero's death, though Harmston feels his survival can, for a modern audience, be made a source of discomfort rather than of sentimental reassurance. He also thinks that, in the handling, say, of Vortigern's wife (who goes mentally AWOL and loses her grip on him and her children), you see distinctive signs of the fact that William Henry Ireland, unlike Shakespeare with his boy actors, was writing the part for a woman. We can judge for ourselves next week - if, that is, the audience, this time round, lets the cast get to the end.

`Vortigern' opens 23 Oct, Bridewell Theatre, Bride Lane, London EC4 (0171-936 3456)