It is the latest in a long list of benefits ascribed to this type of fish.

The research project, at Cambridge University, looked at more than 750 people, some with asthma, and some without.

Oily fish has already been linked with protection from heart disease, arthritis, psoriasis and dementia

Dr John Harvey, British Thoracic Society

They were asked about their diets, and in particular how much oily fish they ate each week.

This revealed that people with diagnosed asthma who were experiencing symptoms such as wheeze, breathlessness, or waking up with a tight chest, recalled eating far less oily fish than those who were recognised asthmatics, but not greatly troubled by their symptoms.

The research, to be presented at the Winter Meeting of the British Thoracic Society on Friday, even suggests that the change in the British diet - often excluding oily fish - could be contributory factor to an apparent rise in asthma rates.

However, government guidance to shoppers says that eating more than one portion of oily fish a week is not recommended because of the risk that their flesh may harbour higher concentrations of heavy metals such as mercury, and dioxin pollutants.

Change in diet

Lead author Dr Bipen Patel, a clinical epidemiologist, said: "The findings are of particular interest as asthma has become more common in the UK over the past 30 years.

"As a nation, the amount of oily fish in our diet has declined over the same period."

Of course, these findings only suggest that oily fish, or the lack of it, could be to blame - many other items have either left or joined the national diet during that period.

Dr John Harvey, the chairman of the communications committee of the British Thoracic Society, said: "This is an interesting piece of research for people with asthma and at risk of asthma.

"Oily fish has already been linked with protection from heart disease, arthritis, psoriasis and dementia - and more research is needed to assess the role of dietary factors in the development of asthma.

"It is safe to say that eating oily fish in moderation every week as part of a well-balanced diet could help reduce the risk of asthma.

However, he added: "I would not recommend over-indulgence since some oily fish can contain dioxins which may be harmful if eaten in large quantities."