Mothers who smoke during pregnancy put their children at greater risk of psychotic symptoms such as delusions and hallucinations, a study suggests.

A UK survey that was recently published in The Daily Mail found that of 12-year-olds those whose mothers had smoked were 20% more likely to suffer such problems. The link was 84% more pronounced if 20 or more cigarettes a day were smoked.

The researchers suggested tobacco exposure in the womb may affect the child's brain development, but admitted further study of the issue was needed. The research by Cardiff, Nottingham, Bristol and Warwick universities was part of a long-running study known as the Avon Longitudinal Study of Parents and Children looking at how genetics and the environment affects health.

During this latest part of the programme, 12-year-olds were asked if they had had any psychotic episodes such as delusions and hallucinations in the past six months. In total, 11% of the group did.

The group was also studied for their mother's use of cannabis and alcohol. No link was found for the drug, while only those whose mothers drank more than 22 units had a higher chance of psychotic episodes.

Researchers said the findings added more weight to the argument against smoking during pregnancy - about 15% of pregnant women still do not give up the habit when they conceive. Lead researcher Dr Stanley Zammit said: "Maternal smoking may be an important risk factor in the development of psychotic experiences in this population."