Mozart's final year was a blur of activity. In retrospect, it seems the composer was almost willing himself to an early grave in terms of the sheer amount of music which flowed from his pen. Not that Mozart had ever been a slouch when it came to productivity, but the year 1791 seemed to bring with it, incredibly, a renewed and even more frenetic burst of creative energy. Often, during this period, Mozart might complete a major composition one morning and begin another the same afternoon; or, as was also often the case, he might work on more than one large-scale piece simultaneously. Indeed, Mozart's final two operas overlapped - he was writing The Magic Flute when he received a commission from Prague for an opera to mark the Coronation of Leopold II as King of Bohemia. The opera which resulted was to be his last - La Clemenza di Tito, composed in just 18 days! For a long time, the received wisdom on the work was that Mozart had expended his best on The Magic Flute and that Tito is a pale shadow by comparison.

"Not so," claims Sir Charles Mackerras (above) who conducts the already acclaimed Welsh National Opera production which arrives at London's Shaftesbury Theatre for two performances this week. "Tito, too, is a masterpiece, though different in many ways to the Flute. In fact, it was the first of any of the Mozart operas to be performed in Britain and was incredibly popular for most of the 19th century. Only in the 20th did people start to wonder why Mozart, at the very end of his life, returned to the, by then outdated, opera seria vein for his final operatic opus."

Why did he? "Two reasons," says Sir Charles. "On the one hand, he was working to a strict commission for this ceremonial work, intended directly for the Coronation. The reasons for the subject matter are obvious - in Metastasio's by then already much-used libretto, the Emperor Titus shows us what a fine ruler he is by dispensing his clemency, even to those who are seeking to bring about his downfall. What better subject for a work marking the coronation of a new Emperor? But, perhaps, even more interesting is the theory that Mozart went back to opera seria to go forward; in other words, that he deliberately exploited the genre to do something new with it.

"In this respect, though they might seem very different on the surface, the Flute and Tito form two sides of the same coin. Both pieces chart journeys from darkness into light; both move from confusion to an Enlightenment notion of clarity and harmony. In fact, it's rather befitting of Mozart's genius that he could achieve a similar effect in two very different genres - a pantomimic singspiel, sung in German, and composed to a new scenario, in which he was working in active collaboration with his librettist, Shikaneder; and, as in Tito, an opera seria, in Italian, using a pre-existing source.

"But the real joy of Tito is Mozart's overall shaping and refinement of his materials - there are some breathtaking and very extended arias, while the piece spreads out from a lavish Overture to explore a range of translucent orchestration, with extensive parts for clarinet and basset horn. To my mind, it's a winner."

Welsh National Opera's production of 'La Clemenza di Tito' opens at the Shaftesbury Theatre, WC2 (0171-379 5399) 7.30pm, 5, 7 March