Babies and young children may be more at risk of getting leukaemia if they are kept in an overly clean and sterile environment, according to a new study.

In some cases this risk factor, coupled with a genetic predisposition, can lead to a child developing the disease after being infected by something as common as flu.

Professor Mel Greaves, from the Institute of Cancer Research in London, claims the most common form of childhood leukaemia - acute lymphoblastic leukaemia (ALL) - could be prevented by "priming" infants' immune systems by exposing them to harmless bugs.

The research, published in the journal Nature Reviews Cancer, is based on a compilation of more than 30 years of work around the world investigating the origins of the disease.

Professor Greaves said: "All the evidence is comprehensive. This is the bottom line for me - ALL is a paradox of progress in society.

"The research strongly suggests that ALL has a clear biological cause, and is triggered by a variety of infections in predisposed children whose immune systems have not been properly primed.

"It also busts some persistent myths about the causes of leukaemia, such as the damaging but unsubstantiated claims that the disease is commonly caused by exposure to elecromagnetic waves or pollution."

The theory could explain why ALL, which affects white blood cells, is more prevalent in affluent societies where infants are shielded from infection.

It may also be the reason why babies that are not breastfed, do not mix with other children in nurseries, are born by Caesarian section, or do not have siblings are more likely to develop the disease.

ALL is rare, affecting about one in 2,000 UK children per year, but can have a devastating impact on families despite a high cure rate.

Prof Greaves' own research points towards a two-step process of genetic changes leading to the disease in children with abnormally formed immune systems.

The first, which may affect up to 5% of children, produces a population of pre-malignant cells but in the vast majority of cases never results in cancer.

Children who have the first mutation are left vulnerable to a second genetic "hit" if their immune system has not developed properly, it is claimed. The trigger for the second mutation might be a common infection, such as seasonal flu, which sets off an uncontrolled inflammatory response.

In laboratory tests, susceptible mice reared in an ultra-clean germ-free environment developed ALL when they were later exposed to common infectious bugs.

Professor Greaves added that parents should "in no way" be blamed for children developing leukaemia, but advised them to be "less fussy about common or trivial infections".

He also urged them to encourage contact between their babies and "as many other children as possible" in the first year of life.

Prof Greaves said: "I hope this research will have a real impact on the lives of children. The most important implication is that most cases of childhood leukaemia are likely to be preventable."