The number of babies born prematurely is soaring and has worrying implications for society, doctors warn. Premature births have increased by more than a fifth in the past decade and are posing a growing burden on families and health services. But doctors do not know what is causing the boom.

The earlier a baby is born before full term - 40 weeks - the more likely he or she is to have physical, behavioural or educational difficulties.

A study in the British Medical Journal shows a 22 per cent increase in premature births - before 37 weeks of gestation - from 1995 to 2004. Among low-risk European women aged 20 to 40 the increase was 51 per cent.

The study was done in Denmark but British specialists said that the populations of the two countries were similar and the same pattern was likely in the UK.

In the BMJ, Andrew Shennan, professor of obstetrics, and Susan Bewley, a consultant obstetrician, at King's College London School of Medicine, say of the increase in premature births: "This is alarming and also implies that clinicians have failed to have any impact on reducing rates."

More than 45,000 babies are born prematurely - before 37 weeks - each year in the UK. They require intensive medical and nursing care to ensure their under-developed organs and body systems do not suffer damage in the first weeks and months of life.

Although severe physical and mental disability is rare, it is more common in those born at fewer than 28 weeks. Even beyond 32 weeks, when babies are likely to survive without brain damage, educational and behavioural problems are still common.

One in four children born between 32 and 35 weeks, less than two months early, requires help in school from classroom assistants.

The burden on the health and other public services is also heavy. Those born at fewer than 28 weeks spend 85 times as long in hospital during their first five years of life. One in four of those born at fewer than 25 weeks suffers severe long-term problems.

Professor Shennan said yesterday: "We do know that there are changes in the population in relation to obesity and other factors. We need to find out what the causes of the increase [in premature births] are because this could be quite devastating for society. We know there are long-term effects."

Obesity and older age of the mother increase the likelihood of pre-eclampsia, a condition in which blood pressure becomes dangerously raised which may necessitate early induction of birth.

But Professor Shennan said that most premature births occurred spontaneously and the causes were poorly understood.

"It is probably related to early maturation of a physiological process involving endocrine alterations, inflammation and uterine stretch with consequent uterine contractions and cervical shortening," the authors write.

A US survey published this week found that women who had less than a six-month gap between pregnancies had a 40 per cent increased risk of delivering prematurely.

Women who left too long a gap, more than five years, were also at increased risk. The survey of 67 previous studies covering 11 million pregnancies was published in the Journal of the American Medical Association.

Charlotte Davies, director of communications at Tommy's, the baby charity, said: "This country has the highest rate of premature births in Europe.

"Until now the medical profession has been unable to recommend women with a time for the body to repair itself. Now that there is a guideline, we could see a marked drop in the number of premature births and low-weight babies. It is important women are made aware of the risks."