The reason may be that the mother passes on immunity to some infections through her breast milk and that other milk contains potentially allergenic components.

Australian doctors studied 2,187 children at ante-natal clinics and followed them up six years later.

Research about the relationship between asthma and breast milk has been equivocal up until now.

Part of the reason, say the researchers, is that many studies have been badly designed.

They believe their research, although inconclusive, strongly indicates a link between feeding children exclusively on breast milk for the first four months and development of asthma and allergies.

They say excluding other milk from the diet for four months was more significant than breastfeeding for a long period.

Dust blamed

The UK has one of the highest recorded levels of asthma in the world and the numbers of people affected is rising.

Some 1.5 million children - one in seven of the population - are estimated to have the condition, compared with one in 25 adults.

Some 2,000 Britons a year die from it and it costs more than £2bn a year in treatment and days off work.

There are a range of different theories about the reasons for the rise in asthma cases.

They include increased pollution and reduced exposure to house dust mites because homes are cleaner.

The Australian researchers looked at other factors thought to play a part, such as maternal smoking and low birthweight.

Asthma costs the UK about £2bn a year

They found that boys, smoking and premature birth were significant risk factors whereas early exposure to childcare protected children from asthma.

But if these were screened out, children who were exclusive breastfed in the early months were 25% less likely to be diagnosed as asthmatic by age six.

They were also 41% less likely to have wheezed three or more times since they were one, 31% less likely to have wheezed in the past year and 42% less likely to have had their sleep disturbed by wheezing in the past year.

Writing in the British Medical Journal, the researchers say further studies are needed, but they say their research could be used to promote the possible advantages of exclusive breastfeeding.

Positive effects

The National Asthma Campaign welcomed the study and said it was "further confirmation that breastfeeding in the first few months of life can reduce the risk of developing asthma as well as other atopic diseases in later life".

It added that breastfeeding had no negative effects, even if the mother was taking anti-asthmatic medication and was worried it would endanger the baby.