Babies born to obese mothers have a greater chance of developing cancer, according to new research.

A greater risk of contracting the disease might be because babies in utero are exposed to more insulin while they are in the womb, scientists behind the study said.

Further, diets high in junk food could also change the DNA of a child born to an obese mother, the study suggested, by changing the "expression" of her genes.

Currently one in five women in the UK are obese when they become pregnant — something already known to increase the risk of the child itself becoming obese and diabetic.

Lead author Dr Shaina Stacy, an epidemiologist at the University of Pittsburgh in the US, said she hoped the study would encourage motivation to tackle obesity.

"Right now, we don't know of many avoidable risk factors for childhood cancer.

"My hope is that this study can be, in a way, empowering and also motivating for weight loss."

The team behind the study analysed almost two million birth and 3,000 cancer registry records, respectively, that were filed in the state of Pennsylvania between 2003 and 2016.

They found children born to severely obese mothers - with a BMI (body mass index) above 40 - had a 57 percent higher risk of leukaemia before the age of five.

This reduces steadily as the mother's BMI falls - meaning cutting down on burgers, cakes and chips during pregnancy may save a child's life.

Dr Stacy said: "Our intent isn't to shame women or make them feel guilty.

"But instead, we are hoping these findings point to one more reason for weight loss."

The results were published in the American Journal of Epidemiology, and were based on the pre pregnancy BMI in mothers and subsequent cancer diagnosis in their offspring.

They held after taking into account other known risk factors for childhood cancer, such as newborn size and maternal age.

The further analysis showed it was not simply larger women were giving birth to bigger infants or that heavier women tended to be older.

Instead, a mother's size independently contributed to her child's risk - which she herself can control.

The researchers don't know why there is such a considerable association between maternal obesity and childhood cancer, but they have some theories.

Dr Stacy said: "We can speculate it could have something to do with disruptions in insulin levels in the mother's body during foetal development, or that the mother's DNA expression could be altered in some way and passed to her offspring."

She added: "But we would need additional studies to glean why that might be the case."

According to the study, not all types of obesity in mothers carry the same level of risk, however.

Among the obese women, those with higher BMI showed with greater cancer prevalence in their children.