Pregnant women who regularly use mobile phones are more likely to have children with behavioural problems, according to a study by a UCLA researcher who had previously written there was no proof of any adverse health effect.

A new study reveals that mothers who used their mobile phone while pregnant were 54 % more likely to report issues such as hyperactivity and emotional problems in their children.

UCLA Professor Leeka Kheifets, who had previously been sceptical of reports linking mobile phone use to behaviour disorders in children, concluded that there did appear to be an association. However, radiation may not be the cause.

The researchers said there may be other possible explanations for the rise in behavioural problems, including that mothers who were frequently on the phone through pregnancy might continue the pattern after birth and spend less time with their babies.

The study of more than 13,000 women found that using a cell phone two to three times a day was enough to raise the risk of behavioural problems in children. Letting children under the age of 7 use a mobile phone also put them in danger.

The UK's Health Protection Agency (HPA) said the study's findings were unexpected and must be taken with caution, but did not advise pregnant women not to use them.

"Its findings need to be investigated thoroughly. There may be another cause for the effect observed," a spokesman said.

The study will be published in the July issue of the journal Epidemiology.