The study looked at the long-term effect of breastfeeding

The University of Bristol team looked at 1,400 babies born from 1937-1939 and followed their progress for 60 years.

Those who were breastfed were 41% more likely to move up in class than those who were bottle-fed.

Experts said the Archives of Disease in Childhood study supported the idea that breastfeeding led to better long-term outcomes for children.

This is a fascinating study which supports the existing body of knowledge that breastfeeding results in better outcomes for babies

Dr Andrew Lyon, Royal College of Obstetricians and Gynaecologists

The people studied had all originally taken part in the Boyd Orr Study of Diet and Health in Pre-War Britain carried out in 1937-1939.

They were followed up until an average age of 73.

Whether or not a baby was breastfed was less to do with class than it is now, when the practice is often more popular with middle-class families.

In fact, there may have been a slightly increased chance that richer families would bottle-feed babies, because they would be able to afford formula milk and nannies.

IQ

The study found there was no difference in breastfeeding rates when the researchers looked at household income or social class.

Those who had been breastfed had a 58% chance of moving up the social ladder compared to 50% of those who were bottle-fed - a relative difference of 41% when the statistics were adjusted to take into account other factors which might influence the outcome.

The longer a child was breastfed, the greater were their chances of upward mobility, the results showed.

And in families where one child was breastfed while a sibling was bottle-fed, there was still a difference in their chances of social mobility, with the breastfed child 16% more likely to move up in class.

Dr Richard Martin, who led the research, said: "We thought that if breastfeeding increased IQ and health in the long-term, it may also have an impact on social status."

But he added: "The question is whether that's an effect of the breastfeeding - something to do with the biological process which has an effect on brain development, or about the activity itself - such as improved bonding with mother, or that people who were breastfed were raised in a better social environment."

Dr Martin said more work was needed to pin down what the explanation was.

Later benefits

Dr Andrew Lyon, a consultant neonatologist and spokesman for the Royal College of Obstetricians and Gynaecologists, said: "This is a fascinating study which supports the existing body of knowledge that breastfeeding results in better outcomes for babies.

"However, the authors themselves admit that the study should be interpreted with caution, and that the findings warrant further investigation before firm conclusions can be drawn".

Dr Mary Fewtrell, a child nutrition expert at the Institute of Child Health, said: "There is quite a bit of epidemiological evidence suggesting that breastfeeding confers benefits for later height, cognitive function and other health outcomes.

"The upward social mobility could be due to an effect of breastfeeding on any of these outcomes - that is, breastfeeding could enable an individual to increase his or her social class by increasing adult height, improving general health or directly increasing IQ and attainment."