Who said that tenors were the preening peacocks of the music world, wrapped up in their egos and fiercely jealous of each other's reputations? Well, me probably. But I take it all back. Two of Britain's finest - Ian Bostridge and Mark Padmore - are about to go head-to-head, singing the same role in different London halls with rival orchestras in quick succession.

The piece is Bach's St John Passion. Padmore sings the Evangelist (the narrator) with the Orchestra of the Age of Enlightenment at the Queen Elizabeth Hall next week, developing the remarkable conductor-less interpretation that he devised a couple of years ago. Bostridge's Evangelist, meanwhile, can be heard in an Academy of Ancient Music performance at the Barbican next month.

But, rather than cursing the competition, the two tenors have agreed to meet me and compare their respective approaches. They certainly come to it from vastly different backgrounds. Padmore is immersed in the Baroque repertoire, whereas Bostridge admits that he hasn't sung a Bach Passion for several years.

“And I've missed them tremendously,” he says. “Going back to when I was at school, singing the Passions was my way of approaching religion. I think that's true of many people. By entering Bach's world we can appreciate something, Christianity, that informs our culture in all sort of ways, but that we have otherwise lost touch with.”

So is the religion of the Passions - the intense, almost visceral way in which they imagine and agonise over the trial and crucifixion of Jesus - the starting-point for musicians preparing the work? “Actually, usually the opposite,” Padmore laughs. “I have performed the Matthew more than 80 times, and can only remember one conductor ever mentioning religion. That was John Eliot Gardiner. People shy away from it, in an embarrassed way. Yet it needs to be addressed. These pieces are not purely musical.”

It's presumably because today's audiences find Bach's religiosity so hard to “address” that the Passions are increasingly being staged as “operas in all but name”. Bostridge has mixed feelings about that trend. “As far as I understand it, Bach despised opera,” he points out. “On the other hand, he wrote music that drew on operatic conventions and had its own strong theatrical logic.”

Padmore is more positive about the benefits of staging the Passions. That's not surprising, since he played the Evangelist both in Deborah Warner's ENO staging of the John, and Katie Mitchell's highly controversial Glyndebourne staging of the Matthew, which set the piece in a school-hall in the aftermath of a Dunblane-like massacre.

“The critical reaction to the Glyndebourne production, yours included, was absolute outrage - which I found very depressing,” Padmore recalls. “But the show divided the audience too. Mostly I found musicians rather hating the experience, and people with a more theatrical imagination getting a huge amount out of it. But then, I think the audience for classical music constantly wants reassurance that the performance sounds like their favourite recording, whereas theatre audiences are prepared to accept much more boldness in interpretation.”

Both tenors feel that they should not be shackled by a preoccupation with performing the Passion exactly as Bach might have performed it. For a start, as Padmore points out, Bach radically revised his score each time he performed it. “He was incapable of writing out a fair copy of any piece without recomposing it. He was completely pragmatic about overhauling his pieces to suit whatever forces were available. That liberates us, I think. The idea that there is this sacred text that we have to honour to the last dot is itself a bit anachronistic.”

Bostridge even doubts whether we know much about vocal style in Bach's Leipzig. “We know that his soloists would have been young: none older than their early twenties. So Mark and I are well past it. But I wonder whether the ‘early music' tradition of using light voices is more about what the late 20th century wanted to hear, rather than what was the case in Bach's day.”

Padmore goes farther. He maintains that we have reached a turning point in our interpretation of Bach. “We have got as authentic as we are ever likely to get without becoming absurd,” he says. “I don't think that looking at, say, mid-18th century Saxon pronunciation of German is going to enhance anyone's appreciation of the music. We are starting to realise that the whole idea of reproducing Bach's supposed intentions, in a museum-like way, is of limited interest to a modern audience.”

Padmore's experiment of doing the Passion without a conductor is one new way of interesting the modern audience. He maintains that, in most Baroque music, “there's no need for a person at the front waving his arms about - it's a 19th-century invention anyway”. Intelligent singers and players, he claims, can sort out the “natural tempo” for each movement. “In fact, I've been in performances of the Passions where it has been obvious to everyone on the stage - except one person - that the music is going at the wrong pace. There's a wilfulness about conductors that sometimes makes them go against what would otherwise be a rather good consensus. I suppose it's their way of showing that they make a difference.”

Bostridge agrees. “The thing about being the Evangelist is that you want to be the conductor as well! You are pacing the story, and it seems odd that you have to deal with this person waving his arms about. I've sometimes been put off even by very distinguished authenticists.”

But, given how little rehearsal time is available in the professional music world, isn't a conductor necessary to ensure maximum efficiency? “Yes, but that's one of my bugbears about classical music,” Padmore responds. “It's being undermined by people not rehearsing enough. We don't have time to ask questions, so we skim the surface. The Bach Choir has been doing the St Matthew in the same way for years. The Novello edition even prints a terrible crib telling you how to rehearse it, which David Willcocks devised. I despair! It's shameful to give great pieces this perfunctory preparation.”

What shaped Bostridge's approach to the Passion? “I grew up listening to Peter Pears as the Evangelist on the Klemperer recording,” he says. “Which possibly explains why I find so many of today's period-instrument performances too fast! The Klemperer recording is absurd; there are all sorts of stories in Dietrich Fischer-Dieskau's memoirs about the singers complaining. But it has wonderful weight. I always find the opening and closing choruses of the Matthew far too fast in modern performances.”

“Try doing it with Colin Davis!” Padmore responds. “I did it with him at the Cheltenham Festival a few years ago - and the performance overran by 45 minutes.”

Mark Padmore sings the St John Passion at the Queen Elizabeth Hall, London SE1 (0871-663 2583), on Feb 24.

Ian Bostridge's performance is at the Barbican, London EC2 (0845 1216823), on March 14

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