The ancient Chinese treatment, which involves inserting fine needles at specific meridians of the body, is a popular complementary therapy for a variety of ailments.

Hugh MacPherson and scientists at the University of York in England said the benefits of a short course of acupuncture were evident in their study of 241 back pain sufferers.

"If you offer acupuncture to someone with back pain on average it is expected you are likely to benefit, not just in the short term but particularly in the longer-term of 12 and especially 24 months," he said in an interview.

"That's a remarkable finding in that normally you would expect the benefit of the treatment to wear off," MacPherson added.

Low back pain is one of the most common medical complaints. It affects ten of millions of people and is a leading cause of sick days.

The researchers compared the impact of adding 10 acupuncture sessions over three months to the normal treatment for back pain, which includes medication, physiotherapy and exercises.

Patient satisfaction and pain levels were measured and recorded during the two-year study. After three months there was not too much difference between the acupuncture group and patients who had the standard therapy.

Weak evidence of improvement in the acupuncture group was found at 12 months, according to the study published in the British Medical Journal.

By 24 months the difference between the two groups increased.

"This is the first study to show this growing gap up to the two-year point. It is quite unique in that sense," MacPherson added.

In a separate study in which they looked at the cost of acupuncture, the researchers found that the additional money spent on the acupuncture treatment appeared cost effective.

The cost of treating each patient in the acupuncture group was 460 pounds ($863) during the study, compared to 345 pounds ($647) for patients who received just the standard care.