Merton and Sutton are home to the highest number of asthma sufferers in London, according to new reports.

Figures released by the Department of Health reveal that Sutton and Merton Primary Care Trust (PCT) treated nearly 20,000 patients for the respiratory condition during 2004/5, the largest number across the capital.

The information has become available for the first time because a new NHS contract means that GPs must now report the number of people suffering from certain diseases and conditions.

A spokesman for Asthma UK said: "Identifying the prevalence of asthma is vital in order to understand how widespread the condition is.

"However, what also needs to be considered is the level of care that is being given to enable people with asthma to control their symptoms."

A spokesman for Sutton and Merton PCT claimed the two boroughs were in line with other London and neighbouring PCTs.

She said: "Sutton and Merton Primary Care Trust is the largest PCT in London. Consequently, the number of patients being treated by their GP is higher.

"However, the number of patients as a percentage of the population shows that Sutton and Merton PCT is broadly in line with other London and neighbouring PCTs and is lower than the national average of 5.8 per cent."

In total the PCT treated 19,553 for the condition during 2004/5.

Health chiefs believe different housing conditions, smoking rates and hereditary factors could all be responsible for making people in some areas more prone to asthma than others.

Experts from London's Imperial College have claimed that living in the capital could reduce life expectancy by 10 years because of exposure to pollution. It is thought dust and other pollutants can affect the respiratory system, triggering asthma attacks in children.