O n July 11 the Royal Opera House will say farewell to its music director, Bernard Haitink , with a gala evening. Haitink has chosen the programme and he will conduct it all, too. There will be the second act of The Marriage of Figaro and the study scene from Verdi's Don Carlos; the evening will end with the finale of Wagner's The Mastersingers. It promises to be an emotional occasion and the goodbyes will be warm and heartfelt, for Haitink, originally from the Netherlands, has become a much-loved and respected figure in British music. With his British wife, home in London and honorary knighthood, he is almost part of the establishment.

More significantly, over the past quarter-century he has occupied three of the most prominent positions in British music. He went to Covent Garden in 1987 after 10 years as music director at Glyndebourne, and before that he was chief conductor of the London Philharmonic Orchestra. If we have seen a lot of Haitink here in the concert hall and the opera house over the past three decades and more, that has been our gain, for he is one of the leading conductors of our age, a superbly natural musician who brings a rare combination of rigour and expressiveness to everything he tackles. Haitink's performances have always been a reflection of the man himself: direct, unshowy and profoundly truthful.

Born in Amsterdam in 1929, Haitink trained in the Netherlands and learned his trade there before becoming the youngest-ever principal conductor of the Amsterdam Concertgebouw Orchestra in 1961. For the first three years he shared the position with Eugen Jochum, but in 1964 he took sole charge of the orchestra, one of the world's greatest. He remained there until his resignation in 1988, combining that post with his work for the LPO and Glyndebourne, carefully nurturing its traditions in what it does best: the 19th-century symphonic repertoire from Beethoven to Mahler.

Haitink has acquired a reputation as a Bruckner and Mahler specialist, thanks largely to the complete cycles of their symphonies he recorded with the Concertgebouw, but his sympathies are far wider than that. He has given outstanding performances of a span of music from Haydn right through to Shostakovich and Britten - including composers not always regarded as part of his territory, such as Ravel and Debussy.

It has been a glittering career on the podium, with the promise of much more to come. Having fully recovered from a serious heart operation four years ago, Haitink is about to take up a new challenge in Dresden, where he becomes music director of the Staatskapelle Orchestra, filling the vacuum left by the sudden death of Giuseppe Sinopoli last year. His position in the conductors' pantheon is already assured, but how positively posterity will view the achievement of his Covent Garden years is a far more difficult question to answer.

For all kinds of reasons, the Haitink years are unlikely to be regarded within the ROH as a golden age on a par with Georg Solti's reign there in the 1960s. Haitink is not by instinct a political animal; he is first and last a consummate musician, and what he does with singers and orchestras is what he regards as his job. But a music director of such a complex organisation as the Royal Opera should have other skills as well, and his unwillingness to get involved with extra-musical matters became increasingly significant during his tenure.

When he arrived at the ROH in 1987, Haitink's operatic experience was relatively limited. He had spent the summers of the previous decade down at Glyndebourne, which then as now offered performers conditions in which to rehearse and perform as near to ideal as any opera house in the world. The coun try-house combination of utter musical seriousness and a relaxed working environment clearly appealed to Haitink; he could immerse himself in the repertory, in which Mozart featured prominently, and also learn how the many variables that go into any opera production had to be balanced and controlled. By the time he settled into his London office in Floral Street, he had conducted a number of times as a guest at Covent Garden, but that was very different from suddenly assuming responsibility for the whole musical output of the house - being its standard-bearer. There were times later during his reign when Haitink seemed to wish that he could wash his hands of the responsibility he was publicly assumed to shoulder.

That detachment hadn't mattered too much at the start. A year after Haitink arrived, Jeremy Isaacs replaced John Tooley as the ROH's general director. The new team, with Paul Findlay as director of the opera company and John Cox brought in to oversee production revivals, enjoyed a honeymoon period. Plans for a cycle of the Mozart/Da Ponte operas directed by Johannes Schaaf came to fruition, although those for a Ring cycle with the Russian director Yuri Lyubimov foundered after the first instalment and a second-hand staging from Berlin by Gotz Friedrich had to be brought in to fill the resulting gaps in the schedule. Haitink's contribution to those projects and to the other productions he conducted was exemplary. He repeatedly did what he had been hired to do: conduct limpid, unfussy readings, which were remarkable for their authority and dramatic grip and which often seemed to increase in understanding and emotional depth through a run of performances. He also demonstrated the knack which all great conductors have of getting orchestras to play at their very best for him.

But the closure of the Royal Opera House in the late 1990s, when the long-delayed redeployment of the theatre finally took place, made fresh demands on Haitink's leadership that were never convincingly answered. The planning for the closure period failed at every level, logistically and financially. Crucially, the plan to find one theatre in the West End that could house the opera performances throughout the rebuilding period collapsed, condemning the company to a peripatetic existence of short seasons in unsuitable venues and a scattering of concert performances. The whole future of the Royal Opera as a coherent institution seemed under threat. If ever there was a moment for a musician of Haitink's stature to come out and demand a solution to the problems, it certainly came then - more than once.

Haitink publicly criticised the ROH board's handling of the closure, and later described that period as the "wilderness years", but at the time he seemed powerless to improve the situation. His intervention on behalf of the orchestra, which was briefly threatened with disbandment, probably kept it intact. But there is no doubt that morale within the company reached a very low ebb, and Haitink did little if anything to improve it.

Perhaps such qualities are beyond the call of duty for a conductor; perhaps a music director is always right to put single-mindedness and an absolute concentration on the musical essentials - the quality of the performance on stage and from the orchestral pit - above all other concerns. Haitink is not a man to nurture a company - to oversee promising singers, to take an overview of what the whole organisation is doing - any more than he is equipped to deal with confrontations, whether they are with besuited administrators and board members or with stroppy singers. He is a serious-minded, dedicated and (I suspect) rather stubborn musician, not one to easily adapt to dealing with the tantrums of overpaid operatic superstars. (The international operatic star system, by which singers often fly in to perform in revivals and sometimes even brand new productions on a bare minimum of rehearsal, was never for him.) If heads need to be knocked together then he is not the one to do it, and there have been occasions when that ability would have been welcomed at the Opera House.

So we will remember Haitink at Covent Garden almost exclusively for his great performances, and certainly there have been some of those. In all he has conducted about two dozen operas during his years at the Garden - roughly comparable with the number his predecessor Colin Davis tackled during his tenure, but hardly the range that Georg Solti commanded there before him, or, one imagines, the highly varied repertory that Antonio Pappano plans for his regime, which begins in September. As a conductor who learned his trade in the concert hall rather than the opera house, Haitink has been careful to stick with what he knows, never straying outside the repertory with which he feels comfortable. With the exception of a clutch of works by Verdi - Il Trovatore, Don Carlos (a regular highlight, in both French and Italian), Simon Boccanegra and Falstaff (with which he inaugurated the refurbished house in December 1999) - he has steered clear of Italian opera. Not a note of the Puccini that features in every ROH season has been conducted by the music director, and pieces by Rossini, Bellini and Donizetti have all been left to others as well.

There have been memorable performances of Mozart's The Marriage of Figaro and Don Giovanni, but not of Cosi Fan Tutte, The Magic Flute or of any of the opere serie . And it is surprising that although Haitink conducted Beethoven's Fidelio during his years at Glyndebourne, he has avoided tackling it at the ROH. A good range of Wagner - Parsifal, Tristan and The Mastersingers as well as The Ring - has mirrored the strengths of his late-romantic interpretations in the concert hall, though Richard Strauss (with the exception of Die Frau ohne Schatten) has been neglected. Borodin's Prince Igor (in Serban's vivid production) and Tchaikovsky's Queen of Spades (in Zambello's pallid one) made me wish Haitink had delved further into the Russian repertory. Smetana's The Bartered Bride, put on during the closure, was not one of the Royal Opera's happiest couple of hours, while the start of what promised to be a Janacek cycle in the late 1980s - with performances of Jenufa, Katya Kabanova and The Cunning Little Vixen, in which clear, clean dramatic outlines were fused with a radiant lyricism - sadly never got any further.

Though Haitink has appeared a few times to conduct ballet programmes - Ravel's Daphnis and Chloe, Prokofiev's Romeo and Juliet and a Stravinsky bill - that is about it as far as opera is concerned. The only unexpected work in his CV is Tippett's The Midsummer Marriage. For a conductor charged with shaping and driving forward the whole musical profile of this country's most prestigious opera company, this has been a desperately narrow repertoire, at a time when the horizons of opera companies and opera audiences have become wider than ever before. It also comes as a shock rather than a surprise to discover that during the 15 seasons that Haitink has been at the helm, there have been only two new operas premiered in the house - Birtwistle's Gawain and Goehr's Arianna - neither of them, it almost goes without saying, conducted by its music director. That is a record the Royal Opera should regard with considerable shame and embarrassment.

When things went right, though, the results could be utterly memorable. There was a wonderfully dark reading of Don Carlos in the old Visconti production that seemed totally coherent and totally focused; a bracing account of The Marriage of Figaro when the Schaaf production opened in 1987 with the young Karita Mattila as the countess; and a sinewy, anguished Peter Grimes in a revival of the ageless Moshinsky production. Most of all, it has been Haitink's Wagner that has lingered in the memory - ever since in his first season he rescued Bill Bryden's desperate production of Parsifal almost single-handedly by his lithe, luminous approach to the score. He has always, I suspect, measured his achievement at Covent Garden by his Wagner.

Eventually, last year, he got the production of Tristan and Isolde he had been waiting for, though what he thought of the late Herbert Wernicke's directorial approach is not on record. And Graham Vick's Mastersingers, a score that brings out the best in Haitink's music-making, was an unqualified success. In one respect, though, the Ring has eluded him: the failure of Lyubimov to deliver the goods in the aborted cycle undoubtedly angered him, and he had to wait until the controversial Richard Jones/Nigel Lowery staging in the 1990s for a production that was made for the ROH.

Then, however, things did not go the way he wanted. His dislike of Jones's approach was widely leaked, though in public Haitink kept his feelings to himself and concentrated on ensuring that musically the shows were delivered to the highest possible standards. Some of the conducting was truly inspired. Best of all was a series of concert performances given during the closure period, when, shorn of the visuals, Haitink's reading carried the drama utterly and unerringly all on its own.

Haitink's failure to respond to what Jones was doing (by all accounts he more or less rejected it out of hand) is surely revealing. For all the dramatic tautness of his conducting, he does not possess a highly developed sense of theatre. The drama within the music he understands instinctively; what is happening on the stage in front of him, though, seems to baffle him as often as he finds it rewarding. It is interesting, too, that the three productions he has singled out for his farewell concert in three weeks' time are in their different ways entirely naturalistic: Schaaf's lively Figaro, the monumental old Visconti Don Carlos (rather than the more stylised recent production by Luc Bondy, which he also conducted when it arrived at Covent Garden from Paris), and Vick's brightly coloured, toytown Mastersingers - nothing too threatening there.

Haitink has always been a musician first and foremost, and a reluctant and wary inhabitant of the opera house. He has continued to work there because he wants to conduct great music - and for no other reason.