Many takeaways and a wide range of snack foods contain them.

In the UK, the use of trans fats has fallen over the past few years, with several supermarkets leading the way by removing them from their own-brand foods.

Trans fats are also found naturally at very low levels in foods such as dairy products, beef and lamb.

But Tam Fry, board member at the National Obesity Forum, said it was the trans fats in processed foods that were the ones to watch.

He said: “This is a question of moderation. You can get away with eating some trans fats that are found naturally in dairy foods and so on. But the trans fats to stay away from in pregnancy and while lactating are those found in processed foods.

“The advice to pregnant women and those breastfeeding is to not throw away the milk and the cheese, because these contain protein, but to avoid these trans fats in processed foods.”

The researchers from Sao Paulo Federal University divided the rats into groups. 

They found babies in the group where both mothers and young had a high fat diet had a greater level of body fat. 

But, crucially, they also found that babies on a control diet were fatter if only their mothers had been given the fat.

The report published in the journal Lipids in Health and Disease, claims there is a direct link between the level of trans-fatty acids eaten by nursing mothers, and the level of these fats in their breast milk.

Researchers also point out that other studies have suggested that babies exposed to trans-fatty acids in the womb and during lactation can be affected into adulthood.

Ms Pisani said: “To date only a few studies have looked at the effects on trans-fatty acids during pregnancy and lactation on the metabolism of offspring in adulthood.

“Fats play a fundamental role in foetal development and changes in dietary fatty acids has important implications for foetal and postnatal development.

“We need to investigate this further as this has important implications for people’s own diets, especially pregnant women.”