Corrective baby helmets have virtually no effect on head shape, according to the most definitive study yet.

The use of rigid plastic helmets has become popular with parents concerned their baby has an odd-shaped head.

But research, in the British Medical Journal, suggests they are little better than doing nothing, and can cause side-effects such as rashes.

Strategies such as placing babies on their stomachs during waking hours should be explored, say researchers.

Most babies go to sleep laid on their backs to reduce the risk of sudden infant death.

Because of this, mild skull deformities – where babies’ skulls are slightly flattened on one side or at the back of the head – are becoming more common.

Except in rare cases, this is generally considered a cosmetic condition.

But some parents seek treatment from a paediatrician, which involves babies wearing a £2,000 helmet for 23 hours a day from six months old until they turn one.

Past studies have produced conflicting results over the success of the therapy.

In the latest research, scientists in the Netherlands followed 84 healthy full-term babies.

Half of the babies wore a custom-made helmet between the age of six months and one year, while the other half had no treatment.

When checked at the age of two years, there was no significant difference between the two groups, with only about a quarter in each group making a full recovery – 26% who had treatment, 23% who had no treatment.

All parents of the babies who had worn helmets reported downsides, including:

However, there was no evidence that helmet therapy influenced the infants’ development or quality of life.

Lead researcher Renske van Wijk, of the University of Twente, said they would generally discourage parents from using the helmets.

“The helmets have no added value compared to doing nothing and a helmet is an expensive treatment,” she told BBC News.

“All parents in the study reported one or more side-effects in the group where infants wore helmets.

“We would discourage helmet therapy in healthy infants, but parents with concerns should always speak to a doctor.”

She added that babies with very severe skull flattening were not studied.

Prof Brent Collett, of the Seattle Children’s Research Institute, said the study was important because it was the first randomised controlled trial of helmet therapy.

“Kids do not appear to grow out of asymmetric head shape, but we don’t know what the significance of that is yet,” he told BBC News.

“At least in this study, treatment did not appear to make a significant difference.”

He said new strategies were needed, such as encouraging the use of “tummy time” – placing babies on their stomachs during waking hours.

Experts say it is vital for parents to continue laying babies on their backs to sleep to reduce the risk of sudden infant death.

Dr Simon Newell, a consultant neonatologist and vice-president at the Royal College of Paediatrics and Child Health in London, said: “Parents should be reassured that they are doing the right thing to nurse their baby on her/his back when put to sleep.

“If this results in a change in head shape, they should rest assured that their child will not come to any long-term harm.”