The BBC's Toby Sealey: Some experts think there may be a link with warmer, centrally heated housing

The UK has among the highest rate of asthma in the world. A 1998 study found that 21% of children under five in Leicester were diagnosed with asthma, compared to 12% in 1990.

The number of children who had suffered wheezing attacks in the last 12 months also increased from 12% in 1990 to 23% in 1998.

Although manageable, the disease causes about 2,000 deaths a year in the UK.

One of the researchers, Professor Mike Silverman, of Leicester University, said: "The findings follow the trend in older children where there has been an increase in allergic disease.

"But asthma in very young children is usually unrelated to allergy, so there is something more fundamental going on.

"Smoking is unlikely to be related because the smoking reported by parents involved in the study slightly decreased over the years."

Prof Silverman said the researchers were exploring the possibility that air pollution from ultra-fine particles produced by diesel engines may be responsible.

The NAC's annual audit of the impact of asthma estimates that one in seven children aged between two and 15 - a total of more than 1.5 million children - has asthma symptoms currently requiring treatment.

In addition, at least one in 25 adults has asthma symptoms currently requiring treatment.

Financial cost

The NAC estimates that the total cost of asthma to the UK is now in excess of £2bn a year.

Many people can control their condition by using an inhaler

Ways to tackle the disease will be discussed at the NAC International Congress, which starts on Tuesday.

NAC chief executive Anne Bradley said: "The vision of the National Asthma Campaign is a world without asthma and from that perspective it is extremely worrying that asthma is increasing in prevalence."

Ms Bradley said tremendous progress in the treatment of asthma had been made over the last 10 years, but efforts had to redoubled.

Sensitive airways

Asthma is a respiratory condition in which the airways to the lungs are almost always red and sensitive.

Asthma attacks are triggered by a violent reaction to viruses, or to other substances to which the body is allergic.

Symptoms include wheezing, coughing, shortness or breath and tightening of the chest.

Scientists are unsure what is responsible for the increase in prevalence of asthma, a complex disease which can effect different people in different ways.

Some people can control their symptoms very effectively using an inhaler, but approximately 200,000 sufferers are registered as disabled.

Antiobiotics may be to blame

It is believed that the comforts of modern living such as double glazing and central heating may be partly responsible.

They create an environment of constant temperature and high humidity which is an ideal breeding ground for the house dust mite, known to be linked to asthma.

Another theory is that people eat too much processed food, and so are not getting enough of the vitamins and minerals needed to help the immune system fight off infection.

The widespread use of antibiotics may also have led to a weakening of the immune response.