The BBC's Nicola Carslaw: Good news about breasfeeding after recent negative publicity

And it found that the longer the period in which babies received breast milk, the greater the benefits, with those breast-fed for a year or longer more than five times less likely to become obese.

The study, published in the British Medical Journal, found that 4.5% of bottle-fed babies were obese by the time they reached five or six-years-old.

But only 2.8% of babies given only breast milk after birth were obese when they reached school age.

The study comes hard on the heels of research showing that British breast milk contains a wide variety of contaminants, some of them toxic.

But it reinforces advice that the benefits of breast-feeding still greatly outweigh any risks.

Researchers looked at 9,357 overweight or obese children from the German region of Bavaria as they reached school entry age, and quizzed their mothers on how they were fed in the months after birth.

Bottle-fed babies are much more likely to be fat

Compared with the 4.5% obesity rate of those children who had never been breast fed, only 3.8% of those who had been fed by breast for just two months became obese.

After breast feeding for three to five months, the likelihood of obesity was only half that of a bottle-fed child.

And less than one per cent of those breast fed for more than a year became obese.

Programming children to be slim

Earlier studies suggest that breast-feeding may help "programme" children not to be obese.

Research found that bottle-fed children had far higher blood concentrations of insulin, the chemical that stimulates the laying down of fat cells.

Obesity while in childhood is known to be a risk factor for developing cardiovascular disease in later life.

Other research has shown that breast feeding for the first 15 weeks protects against both diarrhoeal and respiratory diseases, ear and urinary tract infections and reduces blood pressure.

A spokesman for the National Childbirth Trust said there was strong evidence that breast-feeding protected children's health long after weaning.

She said: "This study reinforces the plentiful evidence demonstrating that breast-feeding is the ideal way of feeding a baby."

The government is still urging mothers to breast-feed, despite launching a probe into the level of contamination affecting British breast milk.

A World Wide Fund for Nature report suggested up to 350 contaminants had been found in various samples, including 42 times the safe level of toxic dioxins - the chemical at the heart of the recent Belgian meat scare.

Other chemicals identified came from suntan lotion and pesticides.