It is the ultimate operatic compliment: a part created just for you. Next week Sir John Tomlinson will sing the title role in Sir Harrison Birtwistle’s new opera, The Minotaur. The retelling of the Greek myth of the half man, half bull is typical Birtwistle: ritualistic deaths; fantastical yet familiar narrative; the mingling of man and beast.

When sleeping, the bull dreams and sings his thoughts, but when he is awake – as he is for half the opera – our creature is incoherent. Trapped at the centre of the labyrinth, he is goaded by a baying crowd: “Speak, freak! Untie your tongue!” But he can only roar.

“Harry is very particular about what sort of roar he wants. He doesn’t want a generalised ‘Uuuuum’,” Tomlinson bellows, sounding somewhere between a donkey and a foghorn. “He wants a peculiar word: ‘Nuuaaargghh!’ I hope I haven’t wrecked your tape machine.”

A fortnight before the world premiere at the Royal Opera House, Tomlinson has heard only a recorded snatch of the orchestra in rehearsal. And he has yet to encounter the finished wire and gauze bull’s head that he must wear for the duration of the opera. For a singer who famously dislikes productions to interfere with the business of singing, this sounds like an Odyssean test. “There are an awful lot of unknowns. Will I be heard?” he says. “Can I see through the head to the conductor?”

The Minotaur completes a Birtwistle trilogy for Tomlinson; Gawain was written with him in mind as the Green Knight, and he has sung Punch and Judy. Birtwistle has said that, while writing this work, he imagined Tomlinson on stage in the lead: “I fully intended it to be a vehicle for him.” He has based the vocal range on Hagen, the murderous schemer of Wagner’s Ring, with an eye for the power of Boris Godunov’s death scene – another of Tomlinson’s defining roles.

Has the bass-baritone become a muse to the composer? “I don’t know about that,” he says. “I’m a channel for his music, both as a voice and personality. But muse implies I’ve inspired him. If he has written a whole piece with me in mind, well, I find that humbling.”

Muse or not, Tomlinson is finding the musical result a joy. “It’s like wearing a bespoke suit,” he says of the score, which pushes his voice just to its limits. The singer is that rare breed – someone who hears genuine beauty in Birtwistle’s spiky, off-beat rhythms and floating vocal lines. The composer whom many consider difficult famously riles audiences. Panic, his saxophone explosion written for the 1995 Last Night of the Proms, provoked complaints to the BBC, while Gawain was booed at its ROH premiere in 1991. Proof, at least, that the classical audience was alive. “There’s blood running through the whole thing with Birtwistle,” Tomlinson says. “Energy, power and great beauty.”

An affinity clearly exists between the two Lancastrians. “Northern qualities have a lot to do with it,” Tomlinson says. “Harry and I share the whole industrial mentality, the ethos. People worked in factories and mines – this is so hackneyed – but the people are forthright and genuine.”

No one who has seen Tomlinson sing a fine Hans Sachs in Die Meistersinger von Nürnberg, Wotan in the Ring cycle, or Billy Budd’s nemesis, Claggart, would question the truth of his performances, a quality he attributes to home. “I was brought up with that honesty: be loyal to things, and if you’re going to do something, do it properly.”

Which brings us neatly to Bryn Terfel. When the Welsh baritone pulled out of Covent Garden’s Ring cycle last year – citing family reasons – with just four weeks’ notice, Tomlinson stepped in to sing Wotan in every cycle. But not before asking Terfel to change his mind: “I said: ‘This is a major decision, please reconsider for your own sake and for everybody’s sake. It doesn’t just affect the Wotan; there are repercussions.’ ” For instance, Covent Garden ended up scrambling three artists to sing Hagen, Tomlinson’s original role.

“There is a dilettante attitude among the big names,” continues Tomlinson, careful to distinguish Terfel from what he sees as celebrity culture infecting opera. “Opera houses have to book these world celebrities ridiculously far ahead [often up to eight years] in order to get them and then three weeks before, the singer thinks: ‘Oh, I’ve decided not to do this.’ The houses can’t trust these people to do their jobs.”

Since his debut at Bayreuth in 1988, Tomlinson has sung Wotan at the Wagnerian festival for an astonishing 18 successive years. In London his heroic efforts on the Covent Garden stage galvanised what had been a shaky Ringcycle, binding the four operas together and focusing the director Keith Warner’s vision. For the first time Tomlinson, 61, hesitatingly acknowledges that this triumph might have been his farewell performance: “Do I want to say this?” He looks away. “Yes . . . in retrospect, they could have been my last complete Wotans in complete cycles, in my life.”

Wotan and the Minotaur: the half god and the half beast are not a million miles apart. Both are tormented by their circumstances – though the Minotaur’s are not of his making. Trapped in his body and in the labyrinth, dreams provide only scant relief from his anguish. “The world outside is lost to you, love is lost to you.” It is beautiful, yet unremittingly bleak.

So thank goodness for Baron Ochs, the great party-boy of Richard Strauss’s Der Rosenkavalier. Tomlinson will sing this role for ENO next month and he can’t wait. “Ochs is the one character I do who is indestructible and optimistic,” he beams. “It’s a joyous part.

“If I want to cheer myself up” – and one suspects that he’ll need to after The Minotaur – “I walk down the street like Baron Ochs and, immediately, life’s just one delight after another.”

The Minotaur opens at the Royal Opera House (020-7304 4000) on Tues; Der Rosenkavalier opens at the Coliseum (0871 4720600) on May 22, 2008

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