The finding will surprise those who believe that the presence of such pets is a trigger, or even a cause, of allergic attacks.

Millions of UK children suffer from asthma and other allergic conditions such as eczema.

Much about the development of the immune system in these children is still poorly understood, and parents are split between those who believe that early exposure to "allergens" will produce tolerance, and those who think children should be protected from them.

The latest study, published in the Journal of the American Medical Association, looked at 474 healthy babies born in the Detroit area.

These were followed up all the way to their seventh year, and the 184 exposed during infancy to two or more dogs or cats compared with the 220 who were not exposed.

Chance halved

They found that children exposed to two or more indoor pets were half as likely to develop common allergies.

The figures were adjusted for factors such as parental history of asthma - which might influence the likelihood of pet owning - but remained statistically significant.

Dr Dennis Ownby, from the Medical College of Georgia, led the study, and believes that clean houses may be to blame for soaring asthma rates.

He said: "The bottom line is that maybe part of the reason we have so many children with allergies and asthma is that we live too clean a life."

Lick of life

He said that early exposure to "endotoxins" - found in the bacterial residue found in the mouth of a cat or dog - could be the protective influence.

He said: "What happens when kids play with cats or dogs? The animals lick them.

"The lick is transferring a lot of Gram-negative bacteria and that may be changing the way the child's immune system responds in a way that helps protect against allergies."

You can't say whether an individual pet will help or harm an individual child

Professor John Warner

Allergic responses are caused when an antibody called immunoglobulin E, binds to a type of blood cell called a mast cell, which then releases the body chemicals which cause inflammation within minutes of exposure.

It is possible that environmental influences at a young age help shape the power of this immunoglobulin E response.

Some scientists have even suggested that it might be possible to produce a "dirt vaccine", containing many different kinds of soil bacteria, to help stifle this immune response.

No worries

British child health expert, Professor John Warner, from the University of Southampton, told BBC News Online that it was a confusing situation for parents.

He said: "To parents who are concerned about very young children, I would say: 'Don't worry about it'.

"You can't say whether an individual pet will help or harm an individual child - your decision should be based on other quality of life factors.

"What is fairly sure is that if you have someone with an existing allergy to cats or dogs, owning these animals will certainly trigger an attack."