Other takeaway food and fizzy drinks also increase the chances of getting asthma, doctors found.

Youngsters who eat at least one hamburger a week are 75 per cent more likely to have asthma and almost 100 per cent more likely to suffer wheezing problems, according to a study published yesterday in the international scientific journal Allergy.

The investigation of the extent to which fast foods are a risk factor for asthma, was led by Dr Kristen Wickens, of the Wellington Asthma Research Group, based at the Wellington medical school.

The group - investigating the role of lifestyle changes over the past 30 years in a big increase in allergic disease - correlated the consumption of fast food to the prevalence of asthma and allergy.

They used 1321 children in Hastings, aged between 10 and 12 years, and recorded their diet, as well as checking asthma and asthma symptoms as part of an international study of asthma and allergies in childhood known as ISAAC.

After adjusting for lifestyle factors, including other foods and how fat the children were - compared with children who never ate hamburgers - they found frequent consumption of hamburgers was linked to asthma symptoms.

And the higher the consumption of hamburgers meant higher incidence of asthma.

Asthma occurs in every country in the world and is worst in developed countries - which tend to have about 6 per cent of their population affected - but New Zealand has the highest incidence in developed countries: 20 per cent.

A greater proportion of the teenagers in New Zealand suffer from asthmatic symptoms, such as wheezing, breathlessness and tight-chestedness.

Some theories have blamed more westernised diets, higher standard of living, falling levels of exercise rates, and increased dustmites and pollution.

But Tokyo, with higher pollution than Wellington has only one fifth of the asthma incidence in Wellington, while the Scottish island of Skye has the highest incidence of asthma in Britain, and almost no pollution.

Dr Wickens, who led the Hastings study, said diets containing junk foods which are high in salt could be contributing to the problem.

"The high salt content in hamburgers may increase the risk of wheezy illness," she said.