Ten years ago, no one dreamed Brian Ferneyhough would compose a work that could be called an opera. Least of all Ferneyhough himself: he admits that until 1997, when he was commissioned by the city of Munich to write a stage work for its Biennale, he had never given the idea much thought. Not that Shadowtime corresponds to conventional notions of what an opera, or even music theatre more generally, can be. The work, which receives its British premiere at the London Coliseum tomorrow, features poetic examinations of the philosophy of Walter Benjamin, and at one point requires a pianist dressed like Liberace to play a furiously difficult solo while reciting texts dealing with alchemy and theories of cognition.

Born in Coventry in 1943, Ferneyhough has lived abroad for more than 30 years, in Germany, Switzerland and, since the early 1990s, California. He has built a reputation, far bigger in continental Europe than in his homeland, as one of the most important of today's unreconstructed musical modernists, famous - notorious, sceptics would say - also for the forbidding density and complexity of his music, which often makes almost superhuman demands on its performers. And in remaining true to the rigorous principles he inherited from the serialists of the postwar generation, Ferneyhough had also shared those composers' deep-seated suspicion of opera, and of all the historical trappings the medium inevitably brought with it.

It was the Munich proposal that really got him thinking about the possibilities of doing something radically different with the form. "The Biennale's stated purpose has always been to encourage new forms of music theatre, so I saw no reason to stick to any prior conventions when drafting the initial concept," he tells me in an exchange of emails about Shadowtime. "Since my initial vision was of an 'opera of ideas' - a playing-out of the 'representation of representation', if you will - there was always the thought that different scenes would address this very 20th-century issue in extremely different ways, ranging from the deadly serious to the wildly parodist extremes of the commedia dell'arte."

There was still the question of what this new concept of music theatre would be about, however. The subject that Ferneyhough quickly settled on was the life (or, more specifically, the death), work and cultural legacy of Walter Benjamin, who killed himself on the border between Spain and France while fleeing the Nazis in 1940.

"Choosing Benjamin as the figure around which everything else was to revolve was not a difficult decision to make," Ferneyhough says. "On the one hand, Benjamin's understanding of the world largely in terms of allegory and epiphany lay very close to my interest in codes of representation; on the other, his own life can surely be read as a sort of morality play in which the strengths and tragic weaknesses of the intellectual and artist throughout the tumults of the past 100 years were only too clearly exposed."

Recruiting a librettist was the next step. A few years earlier Ferneyhough had been impressed by a long poem, Artifice of Absorption, by the American poet Charles Bernstein, whose views on the purpose of art seemed to chime with his own and with what he was hoping to explore in the stage work. By the time he and Bernstein met in San Diego, Ferneyhough says, the Shadowtime project was well advanced, and he had pretty precise ideas about what the text should be and how it should function. "I laid out quite stringent numerical constraints - number of lines, number of syllables in a line and so on - for each unit of the text. I also expressed the wish that, in as many instances as possible, the elements of a particular text would be freely permutable, the better to assimilate them [into] the musical setting. The most important precondition, however, was my wish that the final libretto would also be able to stand as an independently viable poetic work . . . All this functioned perfectly. In fact, it has just appeared as a book of poetry."

Very soon, they had the architecture of the seven sections of the work in place. "It is important to emphasise that Shadowtime is not just a collection of odds and ends around a theme," Ferneyhough says, "but a relatively linear series of takes on the afterlife of ideas." The sequence falls into two parts of three pieces, which hinge around the central work, Opus Contra Naturam, in which the flamboyantly costumed pianist simultaneously plays and declaims. That represents the point at which the journey of Benjamin's philosophy through the afterlife of 20th-century thought reaches the gates to the Underworld, symbolised in Ferneyhough's scheme of things by Las Vegas.

Each of the parts, though, begins with one of the two explicitly theatrical scenes. In the first, called New Angels/Transient Failure (the only time Shadowtime gets anywhere near to conveying a conventional narrative), the events leading to Benjamin's death in a hotel on the Spanish border are depicted. In the fifth, Eleven Interrogatory Instances, Benjamin's philosophy is scrutinised by historical figures, including Hitler, Pope Pius XII, Karl Marx and two of the Marx brothers. The interrogations are represented musically, too, in a sweep through music history that parodies a whole range of styles and ends with six female voices singing in the style of Beethoven's Grosse Fuge. According to Ferneyhough, those two scenes - "the hell of the 'real' world and the floating, surreal utopia of the beyond" - are the "generators of everything else that Shadowtime contains".

Ferneyhough has compared the structure of Shadowtime, with its mixture of instrumental, solo vocal and choral pieces placed in a theatrical framework, with that of the sacred rappresentazione , which at the beginning of the 17th century was one of the most important precursors of fully fledged opera. But anyone who knows a bit about Ferneyhough's career will see parallels between this sequence of pieces, most of which had been premiered as self-standing concert works before the opera premiere in Munich, and the instrumental series that have always been characteristic of his way of working. His three Time and Motion Studies of the 1970s were an early landmark in his music; his major achievement of the 1980s was another cycle of pieces, for forces ranging from a solo instrument up to a large ensemble with electronics, whose title, Carceri d'Invenzione, was borrowed from the artist Piranesi. And in another collection from the 1990s, he explored the various ways of setting up a dialogue between a solo instrument and an ensemble.

Against that background, Shadowtime seems more typical of Ferneyhough. In any other context, the intervening movements - placed second, third, sixth and seventh in the performing order - could stand as pure concert works. The second part, Les Froissements des Ailes de Gabriel (The Fluttering of Gabriel's Wings), is a straightforward guitar concerto, while the third, The Doctrine of Similarity, is a motet-like study in canon for chorus and ensemble. The sixth section, which explores the allegorical aspects of Benjamin's thinking, involves a reciter with the ensemble. And the final one, Stele for Lost Time, attempts a metaphysical reconciliation, using transformations of Ferneyhough's own voice and an invented language.

There's no denying that the content of Shadowtime - the music and ideas behind it - are complex and sometimes hard-going. For anybody encountering this music for the first time, it's best to ignore the density of its intellectual construction and web of symbolism and cultural allusion, and concentrate instead on the beauty of Ferneyhough's music. Underneath the tangled surfaces, he has always been a composer with an urgent desire to communicate, and one who believes passionately in the importance of what he is trying to get across.

He has described Shadowtime as a non-Christian requiem, but when I ask him whether it is a requiem for the western tradition - the culture that produced Benjamin - he denies it emphatically: "It all may be, as most intellectuals placidly maintain, a bit of a lost cause, but that does not absolve us of the duty of relentlessly critiquing our means of understanding the world . . . Who knows, maybe the world might still be altered in some minuscule degree by unfashionable intransigence?"

English National Opera perform Shadowtime in its UK premiere at the Coliseum, London WC2, tomorrow. Box office: 020 7632 8300.