MUCH to the irritation of its rivals, the delight of its audiences and the astonishment of the arts world generally, the London Symphony Orchestra has announced the signing of the year. Valery Gergiev, by common consent the most charismatic maestro in the world today, will be the LSO’s next principal conductor, succeeding Sir Colin Davis in January 2007. From the magisterial to the mercurial, you could say, in one baton change.

Yesterday’s announcement came out of the blue. Gergiev made his London debut with the LSO as long ago as 1988 and conducted the orchestra in an acclaimed concert series of Prokofiev symphonies last year. But negotiations to bring him permanently to the Barbican were the best-kept musical secret for years.

They probably had to be. The 52-year-old Ossetian has eclipsed even Sir Simon Rattle and the Latvian maestro Mariss Jansons as the most exciting conducting talent of his generation. There isn’t an orchestra or opera house on the globe not clamouring to have him on its podium.

He already holds major appointments at the Rotterdam Philharmonic and the Metropolitan Opera in New York. But it is as artistic and general director of the Maryinsky Theatre in St Petersburg that he has made his reputation, both as a stunning interpreter of Romantic opera and as a forceful leader of a venerable artistic institution reborn to new glories.

Exercising a Tsar-like control over every aspect of the theatre’s operation — and wooing Russian oligarchs, Moscow politicians and foreign sponsors with equal success (he refers to Vladimir Putin as “my friend”) — he has not only steered the Kirov Opera and Ballet successfully through the cultural and financial turmoil of post-Soviet Russia, but turned them into powerhouses of dazzling young talent.

How his autocratic style will fit in with the traditional ethos of the LSO — a self-governing band that famously treats conductors as equals, not superiors — remains to be seen. But an orchestra that likes to bracket itself alongside the Berlin and Vienna Philharmonics in the musical pecking order will be delighted to have secured one of the few conductors in modern times with the genuine star-appeal of a Bernstein, a Karajan or a Beecham.

And life is never boring with Gergiev around. On the concert platform he is a dark, unsettling and mesmerising presence. His body is forever hunched forward, as if straining at an invisible leash. His black, bulging eyes flash dangerously. His face, usually bearing a couple of days’ stubble, is stern and lined beyond his years; his hands are perpetually twitching. He gives every impression of a man living on nerves and adrenalin. And his best performances have a similar sort of demonic intensity.

So, indeed, do his worst performances — but in a much scrappier way. That’s not surprising. Gergiev is a man who once conducted three different orchestras in three different countries on the same day. What you get from him is fervour and dangerous thrills, not pristine perfection. Nevertheless, it is remarkable how often his performances do hit the bull’s-eye.

Nothing he does is orthodox. He regularly works a 20-hour day, holding business meetings through the night in a favourite St Petersburg watering hole, then appearing at ten the next morning to rehearse some operatic epic. It is not unknown for journalists to follow him round for days in pursuit of a promised interview, only to be granted one at 4am. But there is usually a large supply of wine or vodka on hand to lubricate the conversation.

His personal circumstances are equally Byronic. Four years ago he married an Ossetian teenage musician nearly 30 years his junior. They now have three children.

The obvious danger of appointing this human whirlwind as principal conductor is that Gergiev’s mind and energies will constantly be focused on about nine different jobs simultaneously, all of them in different time zones. His new, purpose-built concert hall at the Maryinsky opens in 2008. And he shows little desire to cut down radically on his other commitments — which include the artistic direction of half a dozen festivals scattered around Europe.

But if his feet can be nailed to the LSO podium for at least a few weeks each year, he should electrify London’s musical life. Either way, this spectacular signing represents one last coup for Clive Gillinson, the LSO's managing director, who leaves next month after 20 years to run Carnegie Hall in New York. The matter of choosing a successor to Colin Davis (who will have been the longest-serving LSO principal conductor in history by the time he retires, and who will then become the orchestra’s president) had become a pressing concern. But when Gillinson announced that he intended to “tie up all the loose ends” before departing, few realised just what a dramatic announcement he had in mind.