The explosion of interest in the Pilates exercise system — whose advocates include actresses Sharon Stone and Jennifer Aniston and the England cricket team — could be putting people’s health at risk.

A report published today says there is a lack of properly trained instructors and insufficient regulation.

Some osteopaths describe Pilates as one of the safest exercises but only when done. Despite the growing popularity of Pilates, which uses a series of slow stretching exercises and controlled breathing, there is no nationally recognised system for the training and regulation of instructors, it says.

Some private health clubs consider Pilates training courses lasting one weekend as sufficient to qualify as an instructor.

Yoga does have a formal regulatory structure but newer disciplines such as Ashtanga and Bikram Yoga are not included.

Health Which? says it wants to see formal systems of training and regulation for all yoga and Pilates teachers to minimise injuries to people taking part in classes. Nobody should be teaching Pilates or yoga without proper training, regardless of their background or expertise, it says.

A survey of 210 Health Which? readers showed that the majority believed that their teacher was qualified but half could not be sure because they had not asked before attending a class.

People should avoid Pilates classes with more than 10 to 15 participants, said a spokesman. They should also ask to see the instructor’s insurance certificate.

Felicity Porritt, the Health Which? researcher who wrote the report, said: “If learned under a suitably qualified instructor, yoga and Pilates can add so much to your life.

“But the huge surge in popularity in recent years has made it virtually impossible to train enough properly qualified instructors to satisfy demand. It is vital that people check their instructor has sufficient and appropriate training.”

Pilates was devised by German-born Joseph Pilates in the 1920s. He emigrated to America in the 1930s where he opened his first studio in New York for dancers wanting to improve their posture and flexibility.

Advocates say Pilates will greatly improve suppleness and strength while breaking poor posture habits and alleviating back pain.

It is also said to help relieve stress, improve breathing and create a toned body without adding excess bulk, by lengthening, flattening and strengthening the muscles rather than plumping them up.

Some osteopaths describe it as one of the safest forms of exercise because the muscles are worked but not fatigued.

Nigel Stockill, the head physiologist for the England Cricket Board, said: “Aspects of Pilates are used in our training all the time.”

It was particularly beneficial to cricketers because it helped them recover faster from sports injuries and had a strong focus on the abdominal muscles and back, he said.