Mothers who eat junk food during pregnancy may be condemning their children to crave the same diet, according to animal tests.

Royal Veterinary College researchers found that when pregnant rats were fed a diet of biscuits, crisps and sweets, their babies ate more unhealthy food.

They said the British Journal of Nutrition study showed the rats' behaviour was 'programmed' in the womb.

Dieticians have stressed the importance of a balanced diet for mothers-to-be.

Scientists have already shown that, in humans, diet in early life can literally shape your future, setting your risk of obesity and heart disease.

However, the latest research suggests that, in rats at least, eating too much of the wrong food while carrying a child could be potentially harmful.

The female rats used in the Wellcome Trust funded research were either given a balanced diet of 'rat chow' - an unappealing but reasonably healthy diet - or access to as many doughnuts, biscuits, muffins, sweets and crisps as they could consume.

This diet was continued in some rats up to birth, and then during the breastfeeding period until weaning.

Unsurprisingly, the rats given free rein to eat sweets consumed more food overall.

Significantly, however, their babies showed marked differences in behaviour compared with the offspring of chow-fed rats.

The young rats were split into different groups - some of those from chow-fed mothers 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 given junk food again were tempted to eat more.
 
However, the final group - 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 on the junk food or chow menu.

They ate roughly twice as much as those on the chow-only diets.

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

Professor Neil Stickland, who headed the research, said: 'The government is trying to encourage healthier eating habits in school, but this shows that we need to start during the foetal and suckling life.

'Future mothers should be aware that pregnancy and lactation are not the time to over-indulge on fatty and sugary treats on the assumption that they are 'eating for two'.'

However, Fiona Ford, a research nutritionist from the University of Sheffield, said that in the absence of strong evidence that the same effect was present in humans, it would be wrong to make women feel guilty about eating some unhealthy snacks during pregnancy.

She said: 'A balanced diet is important during pregnancy. While this is interesting research, these mechanisms are so finely tuned that I don't think we understand them yet.'

Dr Atul Singham, from the Institute of Child Health in London, also said that he was slightly sceptical about the likely scale of 'foetal programming' in child diet until it could be proven in human studies.

He said: 'This is what we are looking into - but at the moment there is no data in humans to support this, and obviously it is very difficult to carry out intervention studies such as these in pregnancy.'