The researchers arrived at this conclusion when they discovered that pregnant rats who were fed a diet of biscuits, chips and sweets, had young who preferred to eat unhealthy food.

The researchers from the Royal Veterinary College in London say their study demonstrates that the rats' behaviour was 'programmed' in the womb and warn 'mothers-to-be' that pregnancy and lactation are not good times to over-indulge on fatty and sugary treats on the assumption that they are 'eating for two'.

Research has already shown that diet in early life can literally shape people's future, laying down the foundations for obesity and heart disease; dieticians too have stressed the importance of a balanced diet for mothers-to-be.

This latest research however suggests that eating too much of the wrong food while carrying a child could be potentially harmful.

For the study the female rats were either given a balanced diet of 'rat chow', a dull but reasonably healthy diet or had access to as many doughnuts, biscuits, muffins, sweets and chips as they could eat.

This regime was continued in some rats up to birth, and then during the breastfeeding period until weaning; it comes as no surprise that the rats given free rein to eat sweets consumed more food.

However of more significance was that their babies too showed marked differences in behaviour compared with the offspring of chow-fed rats.

When the young rats were divided into different groups, some of those from chow-fed mothers were given nothing but their chow to eat, while the babies of junk-fed mothers, and the rest from chow-fed mothers, were given a mixture of chow and junk food to see which they chose.

Those in the chow-only group consumed the least food, while those from healthy-eating mothers when given junk food again, were tempted to eat more.

However, the babies of junk-food mothers given the option of an unhealthy diet, ate the most food, eating nine days worth of food for every seven days worth consumed by the other babies who were on the junk food or chow menu; they were in fact eating roughly twice as much as those on the chow-only diets.

The researchers suggest that the 'pleasure chemicals' released by the mother when eating junk foods might have an effect on the developing brain of the foetus.

Professor Neil Stickland, who led the research, says the research shows that while the government is trying to encourage healthier eating habits in schools, the process needs to begin during the foetal and suckling stages of life.

Professor Stickland says it is not just about genetics as the study shows a direct link to what the mothers eat and how it affects their children.

Experts warn against making women feel guilty about eating some unhealthy snacks during pregnancy and say that is when a balanced diet is important.

They also say data in human studies are needed to add support to the findings.

According to the World Health Organisation around 400 million people are now obese, including 20 million children under the age of five and obesity is a major issue worldwide as it raises the risk of diseases such as type 2 diabetes and heart problems.

The research was funded by the Wellcome Trust and is published in the British Journal of Nutrition.