In addition, the Oxford Childhood Cancer Research Group found that first-born children face enhanced risks of contracting leukaemia, a chance that diminishes with successive siblings.

These discoveries provide critical insights into discovering the causes of cancer, say scientists. 'This research has taken us a step closer to a comprehensive understanding of the most common form of childhood leukaemia,' said the British cancer expert and Nobel prizewinner, Sir Paul Nurse.

As part of their study, the Oxford group identified more than 10,000 cases of children's cancer from the British National Registry of Childhood Tumours, and compared them with a similar sample of healthy children.

They discovered that children with acute lymphoblastic leukaemia (ALL) - which accounts for a quarter of all childhood cancers - were born more often to older parents. A mother aged between 35 and 39 was 30 per cent more likely to have a child with lymphoblastic leukaemia than one aged 25 to 29. The extra risk rises to 88 per cent for mothers of 40 or over.

'We don't know if this effect is directly related to mothers, however,' said Dr Gerald Draper, a member of the Oxford team.

'Older women tend to be partnered with older men, so we can't be sure whether it is a genetic feature of men or of women that is causing this effect.

'Either a woman's ova are acquiring genetic damage as they age, making their children more susceptible to leukaemia, or the cells that make sperm in a man are acreting genetic damage. Alternatively, it is possible that this effect may be the result of older and younger parents having different lifestyles.'

However, the team stressed their discovery should not discourage older parents from having children. 'Childhood leukaemia is still rare,' said Dr Richard Sullivan, of Cancer Research UK. 'There are only around 300 new cases of acute lymphoblastic leukaemia diagnosed a year. In addition, there has been a vast improvement in treating all leukaemias.'

In the 1960s, only 4 per cent of children with acute lymphoblastic leukaemia survived for five years or longer. By the 1990s, this survival rate had risen to 81 per cent, a remarkable improvement that has been achieved through the development of modern chemotherapy. 'Children's cancers seem much more amenable to chemotherapy than cancers in adults,' added Sullivan.

In addition, the researchers' second, equally intriguing discovery has also shed light on the possible causes of childhood leukaemia. The group found that a family's first born was significantly more likely to develop lymphoblastic leukaemia than later children, with each subsequent child having a lower risk of getting the disease. This result was separate from the effect produced by increasing parental age.

The crucial point is that first-born children are probably less likely to be exposed to infections in early life and this absence of exposure stops their immune systems being properly primed to combat later infections. In a small number of cases, this can lead to the development of leukaemia.

'Our work suggests younger brothers and sisters are exposed to new infectious agents brought into their homes by their elder siblings, and so build up immune defences that protect them against leukaemia,' added Draper.