In the majority of cases, the disease is asymptomatic in both men and women. If symptoms show, they may include discharge or pain when passing urine for men.

Recent research suggests that, if untreated -- even when no symptoms show -- it may be a cause of reduced fertility. In women, it can lead to even more serious complications, such as pelvic inflammatory disease and chronic pelvic pain.

In 2008, young people accounted for two thirds of all new episodes of uncomplicated chlamydia infections diagnosed in genito-urinary medicine clinics in Britain.

In England, as many 68 young men in every 1,000 carry the infection; the figure is nearly 84 out of 1,000 for young women. Since the mid-1990s, the number of diagnosed infections has risen to an average of 7,500 per year to over 123,000.

These findings were published in the Wednesday edition of the British Medical Journal.