Nearly two-thirds of British children are not drinking enough at breakfast time to be properly hydrated, according to a study.

Researchers in Sheffield say the analysis of more than 450 children aged from nine to 11 showed 60 per cent were classed as ''not sufficiently hydrated'' - the stage just below ''clinical dehydration''.

A team from the University of Sheffield Medical School looked at what the children were eating and drinking before leaving for school.

They also measured urine osmolality; the concentration of the children's urine, which is a key indicator of hydration levels.

Professor Gerard Friedlander, of the Descartes University Medical School in Paris, who oversaw the research, said: ''We are concerned by the findings of the study, which suggest that children are not consuming enough fluid at the beginning of the day to be able to maintain adequate hydration through the morning.

''Children are more vulnerable to dehydration than adults due to their high surface-to-body weight ratio.

''They also don't always pay attention to the feeling of thirst, so may not naturally ask for a drink.

''Today we want to raise awareness of the importance of hydration in children and strongly encourage parents and carers to make sure their child drinks enough at breakfast time so that they maintain good hydration, in case they don't drink again until lunchtime.''

Professor Friedlander has overseen similar studies in France and Italy.

He said the British findings closely reflected recent research carried out in France and the United States which showed about 63 per cent of children arrived at school insufficiently hydrated.

The British study showed a higher figure for boys at 68.4 per cent compared with girls at 53.5 per cent.

It was commissioned by Nestle Waters and involved 452 children from 12 schools in the Sheffield area.

The European Food Safety Authority advises that boys aged between nine and 13 years old should take 2.1 litres of fluid a day and girls should get 1.9 litres.

It recommends children at this age drink at least eight 150-millilitre glasses of water a day, slightly smaller than the glass size recommended for adults.

Child psychologist Pat Spungin said: ''Although it can sometimes be tricky to get children to drink water, the key is to encourage drinking little and often.''

They should have a glass of water before school.

Dr Spungin said: ''Plain water should be the first choice for all day-long hydration.''

Professor Friedlander said children who are not sufficiently hydrated at breakfast time are at risk of becoming dehydrated, possibly by lunchtime.

He said some studies done into the effects of lack of hydration in children show it can affect mental performance, such as concentration, short-term memory and attention.

More research was needed in this area.

The professor said it was known that even mild to moderate dehydration can cause tiredness, headache, a dry mouth, decreased urine output and even stop tears when crying.

Professor Friedlander said it was also important children develop good hydration practices at an early age to avoid potential health issues in adulthood, such as kidney problems. PA