Sting/Karamazov/Stile Antico, Lso St Luke's, London

Ever since early music's Pied Piper David Munrow hit the small screen in the Seventies, the lute has steadily moved up the charts as the thinking man's guitar. And Sting is, by rock standards, the quintessential thinking man: his route to the lute began 20 years ago when a friend propelled him towards Peter Pears and Julian Bream doing John Dowland.

At that time he couldn't see any mileage in it for himself, any more than he could when Katia Labeque suggested that Dowland's songs might suit his "unschooled tenor", but he was drawn ever deeper into what he calls the "labyrinth". Stuff kept happening: he was given a custom-built lute with a labyrinth-shaped rose-hole, and he stumbled into a collaboration with the Sarajevan lutenist Edin Karamazov.

And, after years of patient readjustment of brain and fingers to this guitar-which-is-not-a-guitar, Sting has now unveiled a CD entitled Songs from the Labyrinth, consisting of Dowland's loveliest works interspersed with lute fantasias and extracts from the composer's letters.

Which brings us to the LSO's beautifully converted Hawksmoor church, together with ticket touts, rock celebs, the early-music fraternity and Fiona Talkington, who explains that what we are about to hear will shortly reverberate round every corner of Radios 2 and 3. Taking the stage with Karamazov, Sting modestly remarks that he'll be singing in his own style, "but one hopes in a respectful way". And he has a pertinent thought to pass on: Dowland's, he says, is "the music of self-reflection, which leads to melancholy, which has nothing to do with depression, and is an undervalued emotion in modern times". Well put.

Whereupon he launches into "Flow My Tears". Having listened to the CD, I've been looking forward to this moment: Sting has none of the preciousness one associates with amateur shots at Dowland, and his mildly transatlantic accent forces me to listen to the music as something entirely new. But what a shock when he opens his mouth: shorn of that studio mastering, which turns recorded dross into gold, his sound is awful, by turns a croon and a rasp. And he's trying so hard, in all the wrong ways, that it positively hurts to listen.

For this church has an unforgivingly faithful acoustic, and Dowland's melodic lines have the beauty of simplicity: Sting is stripped naked. In one pre-concert interview he argued that since this music pre-dated opera, his lack of "technique" didn't matter. That may be true, but we're talking here about something more universal than mere bel canto - about an artistry that spans everything from Big Bill Broonzy to superstar tenor Ian Bostridge, who just happens to be sitting in my row. And poor old Sting, for all his good intentions, simply hasn't got it. It's worst when he attacks one of Dowland's long-held high notes, but it's pretty bad throughout. The only time it becomes bearable is when he roughens up the street-cry in "Fine Knacks for Ladies", or hardens his tone against his moody mistress in "Clear or Cloudy". I begin to long for Bostridge - or any decent folk singer - to replace him, and demonstrate what good singing is.

The concert doesn't last long - just the length of the CD - but when the celebs have given him their ritual ovation he picks up his lute, turns it into a guitar, and launches into one of his own songs - and we realise that he can sing after all. Performing for 5,000 people is a breeze compared with this, he explains: "I was never so nervous in my life." Now he, his instrument, and his material are one, and we all breathe a collective sigh of relief: don't give up your day job.