At the end of a 150-page chapter on Don Carlos, examining the five versions through which the opera passed in Verdi's lifetime (to say nothing of the various permutations between them that have since been adopted by editors and conductors), Julian Budden, in The Operas of Verdi (Cassell: 1973), asks which is the ideal form in which to present the work but concludes 'Alas, there can be no simple answer.'

On the stage, Claudio Abbado agrees, any dramatically cogent solution to the problem must involve omitting some fine music (there are, for example, two alternative versions of the opera's opening scene, and two of the Act 5 finale) and, probably, a very long evening in the theatre as well, as the composer acknowledged by permitting the 1884 Milan version of the score, which removed the First Act – the Fontainebleau scene – entirely. 'It is very difficult. When I conducted Don Carlos at La Scala in the 1968-9 season, we used the final five-act version, the Modena version of 1886, but in the 1976-7 season I added the long introductory scene that Verdi cut before the first performance in Paris and the duet and ensemble at the end of Act 4, after the death of Posa, which he replaced with a much shorter finale in 1884. You know, even the four-act version is quite a long opera (though how can it be 'long' if it is good?), but it is all such beautiful music that audiencies should have the opportunity of hearing it. On a recording we can have what I think is the best solution: we are using the Modena score, complete, together with an appendix of music from the Paris version that Verdi cut out or replaced: that opening scene, for example, the end of Act 4 (we know part of that already: Verdi re-used it in the Requiem), the duet in Act 3 where Elisabeth and Eboli exchange dresses, and the whole of the ballet music. And we are recording the opera in French.' (But why with mainly Italian principals? a mild question by way of reply: 'Just think: are there any good French singers?') 'Verdi wrote it in French, his music always comes from the words, the sound of the words, and the Italian translation is awkward – Verdi himself wasn't happy with it. Of course some of the music we're recording was never translated at all; also sometimes the music has been changed for the Italian words - little things, yes, but they really don't follow the line of the music.'

The preservation of the whole of the Fontainebleau act, Abbado feels, is important both dramatically and musically. 'It is a great act, and you need it to understand the drama. Without it Don Carlos and Elisabeth are no longer the principal characters: King Philip, Posa and Eboli all have more music than they do. Like the St Basil Cathedral scene in Boris Godunov it also confronts the royal characters with the people. And then think of the appoggiatura that Verdi uses throughout the opera, whenever there is sadness, whenever there is loneliness, like Philip's monologue in Act 4 - in the full version of Act 1 the whole work begins with that appoggiatura, low in the strings. It's part of the characteristic sound of Don Carlos, together with those bass voices - Philip, the Grand Inquisitor, the Monk - and Verdi's use of low strings, contrabassoon and so on: it's a very deep opera.'

Although at La Scala Abbado has tried inserting music from the Paris version into the 1886 score he has not so far performed it there in French. 'The great difficulty is for the chorus to learn it in French. The soloists have more time to do this, and besides, Ruggero Raimondi, our King Philip, lives in Monte-Carlo and speaks very good French, so does Lucia Valentini Terrani, who's singing Eboli, and for the other singers we have a French coach, Janine Reiss.' (She, incidentally, has her own music-stand on the platform in the recording studio with the soloists, and is kept very busy advising on those so un-Italian vowel-sounds and persuading the chorus not to abandon the French 'heureux' in favour of something much closer to 'ere' the moment they turn their backs on her - which they do frequently while recording the Fontainebleau act, in order to give a suggestion of distance.) Abbado stoutly denies that Italian singers think French is bad for their bel canto, but don't they really mind all those eu, an, en sounds? 'Oh, maybe they mind...'

Meanwhile Katia Ricciarelli, singing the role of Elisabeth, stares fixedly at Janine Reiss (who mouths silently back at her) in a fierce attempt to remember that in French the name of the tenor hero she is so passionately addressing ends in 's' (Carlos, not Carlo, and the accentuation is different, too), and a comprimario singer, announcing 'Le Grand Inquisiteurrrrr' with a flamboyantly rolled 'r', is derisively echoed by a concerted trill from the brass section. 'Italian orchestras,' says Abbado, choosing his words with care, 'don't normally have great discipline, but this one has great concentration: when it comes to a take, they give.' In fact when I arrived, by mistake, half-an-hour early for a session, most of the orchestra (a conspicuously young orchestra, by the way; some of them recruited from Abbado's European Community Youth Orchestra) was already there and practising.

The concentration that Abbado is so conscious of is necessary in recording: it is striking to watch orchestra and singers repeating a build-up of tension. At one point, for purely technical reasons, the climactic passage of the Act I duet has to be repeated no fewer than six times. Each time Ricciarelli reproduces the effect of long-pent intensity, each time Plácido Domingo (Don Carlos) turns to her encouragingly and she smiles back with slightly nervous relief; each time it sounds perfectly fine to me. Again one is reminded of what different lives singers lead from the rest of us. Necessarily polyglot, they joke among themselves in three or four languages at once. They live to a different clock: the sessions run from 3pm until 6pm in the afternoon and from 8.30pm until 11.30pm in the evening, and when Domingo suddenly proposes pasta at 6.15pm it is not an outlandish request, simply lunch. As a session slowly proceeds, they can be kept for hours waiting to sing, but must then be instantly ready. Because of a last-minute cast change and a consequent rapid re-drawing of the schedules Leo Nucci (Posa) sits around the studio for eight hours and is finally not required to sing at all that day; he smiles good-humouredly and goes home. Ann Murray (Thibault), having driven from Munich (in order to spend a few hours en route with her husband Philip Langridge - himself on the way to Monte-Carlo for the Don Giovanni rehearsals for which Raimondi is already overdue) eventually gets to sing a line and a half, approximately, at 11.15pm.

That day's sessions were planned to cover six-and-a-half minutes of Act 1, just over three minutes from Act 4 and nine minutes from Act 2. 'The public don't hear this,' says Abbado with grim humour, 'and it's better that they don't. The singers have to change character, to grow older or younger, between takes – terrible!' He minimizes both the troubles this causes him - of coordinating the tempos of adjacent sections recorded weeks apart, say – and the remarkable skill needed to cope with it ('the tempi are always in the head: I try to concentrate on the music and forget about mistakes!') but he dearly prefers to record complete performances, even live ones. 'I have recorded Berlioz's Te Deum live, and Mahler symphonies. When you are doing a live performance there is sometimes something magic that happens. In the studio it is...different; it is...' Unnatural? 'Maybe. Sometimes it is not musical. But that's the way now, and it's hard to say "I'm not going to record any more".' Especially hard, perhaps, when recording this opera: 'it is one of the best of Verdi's operas, very deep, very modern - wonderful. And pure Verdi: he wrote it for Paris, yes, but no, you're wrong, there's no Meyerbeer in it at all.'

Nor does he see any problems in the much criticized ending, of which he prefers the earlier, longer, quietly mysterious version. Is it the ghost of Charles V that appears, or the Monk disguised as Charles V? 'Oh, it is Charles V, for Verdi it was Charles V' (true, the Emperor abdicated, Titurel-like, in favour of his son, and lived in seclusion in the monastery where Act S is set; he died when the historical Don Carlos and Elisabeth de Valois were both 13 years old but, as Verdi very well knew, '[the real] Don Carlos was a fool, a madman, an unpleasant fellow. Elisabeth was never in love with Carlos. Posa is an imaginary being...in this drama there is nothing of history'). And Abbado approvingly quotes Toscanini: '"One of my little mistakes was not to conduct Don Carlos often enough" - I shall not make that mistake.'