Dr Ashok Rajgopal says he has performed knee replacement surgery on a number of leading yoga gurus.

His warnings are a serious challenge to those who say yoga, which is now a multi-billion pound global industry, can ward off the effects of ageing and leave devotees feeling fitter, stronger and at peace with the world.

Some of its most charismatic teachers, like India's Baba Ramdev, who has built a worldwide empire through television appearances, believe its breathing exercises can even cure diseases like HIV Aids and cancer. In the United States alone, more than 4 billion pounds a year is spent on yoga equipment and 15 million people are regular practitioners.

But according to Dr Rajgopal, the extreme stretching exercises at the heart of the discipline cause severe stress on joints, leading to arthritis.

He has seen a higher incidence of joint and bone ailments among yoga followers.

"Yoga is wonderful provided it is done in a controlled environment, and people are trained and built up to doing such postures but putting the public at large through these extreme yoga postures can create problems for them," he said.

"Many yoga gurus had to undergo knee surgeries, they had been affected by yoga postures," he told The Daily Telegraph.

"Extreme postures like acute deep knee bends, particularly for people who are not used to doing these postures, it is definitely harmful to them in terms of the abnormal stresses, and damage to cartilages. In that respect it is harmful."

He said he had seen particular problems among people who had practised yoga in classes of more than 100 people.

"We see a significant number of people who have been not trained adequately to get into yoga and harmed themselves.

"The vast majority of people have to build up to the level where they can actually take to such postures without harming their joints."

Many yoga gurus had suffered knee and joint problems from performing the 'vajrasana' posture, also known as the 'thunderbolt,' where the practitioner kneels with heels tucked under buttocks, while he or she performs a 'pranayama' fast breathing routine.

Savira Gupta, an instructor at India's Yogalife centre said while it was possible to suffer injuries in yoga, they could be avoided by slowly building up to more strenuous exercises.

"Anatomy is key when you are teaching yoga because everybody has a different body and build. We have to be very careful how we could keep up from one posture to another without injuring them. Everything has to be done according to what your body can handle. With proper alignments and training one can avoid these injuries," he said.

A yoga society was formed in Britain in 1910 but its popularity began to take off in the late 1950s when pioneer BKS Iyengar taught Sir Yehudi Menuhin and later in 1968 after the Beatles joined a yoga retreat in India with their guru Maharishi Mahesh Yogi.

In the 1970s its popularity spread through school halls and community centres as 'keep fit classes.' Today there are an estimated 30 million people around the world practicing yoga every week with around 500,000 in Britain where there are more than 30,000 yoga classes.