The opera world is in turmoil. The Royal Opera House, Covent Garden has committed a terrible crime-one that so offends against operatic received wisdom that opera critics across the planet may never be able to bring themselves to forgive it, or to cross Covent Garden's portals again. It has fired a singer because she is too fat.

Deborah Voigt, who is thought to weigh some 220 pounds, has one of the most impressive dramatic soprano voices around. She was due to display it at the Royal Opera House this summer in a production of Richard Strauss's "Ariadne auf Naxos." Unfortunately, however, she would have had to display far more than just her voice: The production requires her to wear a slinky little black dress. In Ms. Voigt's case, the word "little" was something of a problem, and the director decided that the mise en scène could not survive such a sight.

Cue uproar! On and on it has gone, as the Royal Opera House has been subjected to a barrage of verbal assaults from the arbiters of operatic taste, who have condemned her dismissal as the ultimate triumph of style over substance. There is, we are told, only one thing which really matters: the quality of a singer's voice. The rest is superficial, a blight of the modern obsession with looks and image. As the New York Times' critic, Anthony Tommasini, put it yesterday, "The Royal Opera would seem to have forgotten the most basic truth of the genre. Yes, opera is a form of drama. But drama in opera has never been dependent on literal reality."

Indeed not. But when so many of its most passionate supporters (of which I am one) live in an alternate universe, refusing to recognise how weird is their sense of aesthetics, then it's little wonder that opera is, if not a dead, then at the very least a dying art form.

A couple of years ago I saw Pavarotti's last performance at Covent Garden, as Cavaradossi in Tosca, the role he has returned to this week for his farewell appearance at the Metropolitan Opera in New York. To describe his London performance as embarrassing is not even to come close to reporting the cringing which seized the audience.

His voice remains remarkable, given that he is nearly 70. And yes, it was a great pleasure to hear him sing music at which he excels. But to watch? As the Metropolitan's audience are seeing, so fat and immobile is he now that, to prepare for his final moments, when his character is shot by soldiers, he laboriously manoeuvred himself in front of a large pile of soft pillows, which just happened to be next to him at the top of the Castel Sant'Angelo-well, they would, wouldn't they?

After being shot to death, he carefully bent down and lay on the pillows, rather as if he was preparing for a gentle nap. Literal reality it was not. I do not exaggerate when I say that the sound of laughter from the audience almost drowned out the music. If he had sung the role in a concert performance, the voice would - quite rightly - have been all that mattered. Onstage, it was a tawdry, grotesque, awkward evening, which served neither the composer, the music, the singer nor the audience.

Indeed, the very role from which Ms Voigt has been fired, Ariadne, was performed two decades ago at Covent Garden by Jessye Norman at the height of her powers. Ms Norman is also a large lady. So large that when she came to embrace her fellow performer, Kathleen Battle, Battle disappeared from view under Norman's flowing robes. The image remains my lasting memory from the evening, rather than the quality of her singing.

When Ms Voigt performs Sieglinde in Wagner's "Walkure" at the Met later this month, she will no doubt triumph, musically and theatrically. Neither the role nor the production require her to look one way or another. But sometimes-as in the Royal Opera's production of "Ariadne"-they do. It's as simple as that. If that means there is an even scarcer pool of talent on which to draw, so be it. Singers such as Renee Fleming, Karita Mattila and Angela Gheorghiu show that it is possible to both look and sound sensational.

The whole point of opera is that it is a theatrical experience. Yes, it is possible that Mimi, the consumptive heroine of "La Boheme," could be sung sensationally by a soprano who looks as if she once competed as an East German shot putter. If you close your eyes, it can work. But opera is about sight as well as sound, and it's time that those critics who think that producers and directors are parasites on the otherwise pure operatic body learn that both hearing and seeing matter.