It's hard, it's fast and it tones tired bodies. But, Andrew Johnson and Marged Richards ask, was yoga ever supposed to be a competitive sport?

There is chanting and meditation. And there are strange, contorted poses called the lotus and the frog. But when the latest yoga craze sweeps into Britain from the US later this year, it will not be ancient art as we know it.

There is chanting and meditation. And there are strange, contorted poses called the lotus and the frog. But when the latest yoga craze sweeps into Britain from the US later this year, it will not be ancient art as we know it.

Backed by the likes of Sting, Christy Turlington and actress Uma Thurman, a "power" version called Jivamukti has already taken coastal America by storm and threatens to do the same here. Sting is already lined up to open Britain's first specialist centre in West London which describes its exercises as "walking the razor's edge".

But Jivamukti, an aggressively modern take on the traditional discipline, practised to the music of Miles Davis or John Coltrane, has already upset the karma of the normally tranquil world of yoga.

Purists dismiss it as a "supermarket" version of the practice with macho, competitive elements that contradict yoga's most basic principles. There have even been claims that, in encouraging beginners to try and push their bodies into advanced positions, this and other "sport" versions are actually dangerous and can cause injury.

Such criticism has not harmed Jivamukti in New York and Los Angeles, where fans have been flocking to the 90-minute classes, attracted by its ability to tone up tired bodies, as well as by the impressive list of celebrity endorsements.

Where traditional yoga, as practiced by Mick Jagger and Stella McCartney, encourages slow movement from pose to pose, Jivamukti is described as a "tough" practice, and participants work up a serious sweat.

The US Jivamukti centre says: "Classes are designed to be physically challenging. Challenging your preconceived notions about what you think you can or cannot do helps you push beyond the limits imposed by your mind."

Jivamukti was developed by Americans David Life and Sharon Gannon. In his foreword to their book, Sting writes: "David and Sharon have inspired and encouraged us to think of yoga not just as a system of exercises but as a door to the infinite."

Inga John, who is helping launch Jivamukti in Britain, said: "It's very invigorating. It's a tough practice, done to uplifting music - a very important element is the music and the chanting. David and Sharon made it edgier and more interesting to a modern audience.

"It is a vigorous physical practice, but it is also very graceful. It's the hottest exercise at the moment and it does make you look good."

Gary Carter, who runs the Natural Bodies yoga centre in Brighton, recently named one of the 10 best centres in the country by The Independent, said: "We tend to see people coming in with a number of injuries from these newer forms of yoga, which we have to unravel. Injuries are to the neck, shoulder, toes, knees, hips and lower back. They are pulling their bodies into positions they are not ready for.

"For some, aerobics has been replaced by the hardcore branch of yoga and the attitude is the same as going into a gym," he said.

John Stirk, one of Britain's leading yoga teachers, who is also an osteopath, said: "I've seen many people who have injured themselves by pushing themselves too hard and too far. A lot of bodies don't need things done to them, they need things undone. They should follow the body, not work against it.

"Yoga generally has an enormous amount to offer. It's a shame that people who take this on often don't take the maximum benefit. They do incur injuries. Everyone should do yoga, but at their own pace."

Jackie Barker, a yoga teacher from Oxford with more than 30 years' experience said: "This is a young person's yoga. There are a lot of males - it's a bit egotistical. What is dangerous is that people aren't aware of what they are doing. You can quite easily end up getting injured."

Others, however, are more welcoming. Liz Lark, who has published several books about yoga, said: "Those power yogas are very big in LA. It's about a sweat- loaded atmosphere. It can attract people who want to be 'good at yoga', the antithesis of what yoga is. And it can be a bit too fixated on physical achievement.

"When people start yoga they often start with the physical, then find they are relaxed. It's good to learn postures slowly and carefully with an experienced teacher."

Jivamukti denies absolutely that it is in any way dangerous. "There are different levels of yoga and newcomers are always encouraged to go to a beginner's class," said a spokeswoman.

"The teachers are well trained and encourage you to learn the basic moves. You're doing very natural body movements, no force, just natural posture."