Mothers who eat a junk-food diet in pregnancy may seriously damage the long-term health of their child, according to research published today.

The study, funded by the Wellcome Trust, was carried out on rats, but scientists say they have every reason to believe that human babies may suffer as much harm as the offspring of rodents from exposure in the womb to high levels of fats, sugar and salt.

Last year the same team, from the Royal Veterinary College, London, published work showing that rats fed a diet of junk food were likely to have overweight babies that had a preference for junk food.

The new study, published today in the Journal of Physiology, finds that the bad effects are lasting. Even when the baby rats have been weaned off junk food and on to a normal diet, they are likely to grow up fatter than normal and with a raised risk of heart disease and diabetes.

"It seems that a mother's diet while pregnant and breastfeeding is very important for the long-term health of her child," said Dr Stephanie Bayol, one of the report's authors. "We always say, 'You are what you eat.' In fact, it may also be true that 'You are what your mother ate.' This does not mean that obesity and poor health is inevitable, and it is important that we take care of ourselves and live a healthy lifestyle. But it does mean that mothers must eat responsibly while pregnant."

Bayol and her colleague, Professor Neil Stickland, fed one group of female rats on processed food such as doughnuts, muffins, biscuits, chocolate, crisps and sweets during pregnancy and lactation, while a second, control group were given a healthy diet.

The offspring of the rats fed on junk food had raised levels both of cholesterol and triglycerides, a type of fat found in the bloodstream. Both raise the risk of heart disease. They also had higher glucose and insulin levels, which increase the risk of type two (obesity-related) diabetes.

Once they had reached adulthood, the rats whose mothers ate junk food were still fatter than those in the control group. Most worrying, excess fat accumulated around the kidneys. This perirenal fat is involved in the development of insulin resistance and type 2 diabetes.

The researchers were surprised to find that there was a gender difference - males whose mothers ate junk food had higher levels of insulin and normal levels of glucose, so running a greater diabetes risk, while females had higher glucose levels and were fatter.

Some human epidemiological studies have come up with findings that support the team's work. Women who are overweight have been shown to be more likely to have an overweight child.

Dr Pat Goodwin, head of pathogens, immunology and population health at the Wellcome Trust, said: "This study supports the idea that there are many different risk factors that can lead to someone being overweight and developing related health problems. Pregnancy can be a difficult time for many mothers, but it is important that they are aware that what they eat may affect their offspring."

Jane Brewin, chief executive of Tommy's, the baby charity, said: "This new research supports an increasingly compelling argument that obese pregnancies could further compound the occurrence of obesity in future generations by 'pre-programming' the baby in the womb.

"As it stands in the UK, obesity has already reached epidemic levels and the government is estimating that by 2020 around a third of adults will be obese. Obesity poses one of the biggest threats to maternal and foetal health today."