Women who smoke when they are pregnant are storing up health problems for their children, researchers say.

Cigarette smoke damages the unborn babies' lungs at crucial points in their development, leading to reduced lung capacity in later life, they say.

A team of scientists at the University of Southern California said passive smoking after birth also had an effect, but it was not as marked as the damage done during pregnancy.

They tested the lung capacity of 3,300 children aged between 10 and 16 and asked them and their parents to complete questionnaires about smoking habits before and after birth.

The children's lung function was also tested using a spirometer, an instrument for measuring the volume of air which can be expelled from the chest after the deepest possible breath.

Just under 20% of mothers had smoked during pregnancy and almost 42% of the children were exposed to smoke at home after birth.

In the UK, the percentage of women who smoke during pregnancy is 25%, rising to around 30% in London.

The South California team found the airflow was significantly impaired in the small airways of children whose mothers had smoked.

The effect of environmental smoke added to the damage already done while the child was in the womb - in utero - said the team led by Dr Frank Gilliland at the university's department of preventative medicine.

He warned that the long term effect for children whose mothers smoked during their bodies' development could be obstructive pulmonary disease, lung cancer and cardiovascular disease.

Lung diseases such as bronchitis and emphysema are characterised by shortness of breath. Lung cancer and cardiovascular disease are the leading causes of death in industrialised countries.

'Particular concern'

Dr Gilliland said in the journal Thorax: "The long term effects of in utero exposure on the growing lungs of children are of particular concern. In utero exposure is associated with lower lung function in childhood.

"Reducing the burden of chronic respiratory diseases associated with tobacco smoke may require the reduction of smoking among women during their child-bearing years."

Janet Stocks, professor of respiratory physiology at the Institute of Child Health in London, said tackling smoking during pregnancy was a key requirement.

She said: "This study showed there was an effect of environmental tobacco smoke, but it was minimal once you took into account the children's lungs were probably altered during this vital stage of their development.

"There is this awful concern that the prevalence of maternal smoking is so high. There has really been no change over the past 10 years despite all the health education."

Clive Bates, director of Action on Smoking and Health - said: "When babies are exposed to the chemicals in cigarette smoke while still in the womb, it is just about the nastiest form of passive smoking imaginable and it looks as though it does lasting damage.

"It's hard to think of a more pervasive and pernicious assault on the health of the most vulnerable infants."

But he warned against blaming mothers, and said the NHS needed a strategy to deal with the problem.