Nearly two thirds of children are not drinking enough at breakfast time to be properly hydrated, according to a study.
Researchers in Sheffield believe the analysis of more than 450 children between nine and 11 is the first of its kind in the UK.
It showed 60% 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."
Prof Friedlander has also overseen similar studies in France and Italy.
He said the UK findings closely reflected recent research carried out in France and the United States which showed 62.2% and 64% respectively of children arrived at school insufficiently hydrated.
The UK study showed a higher figure for boys at 68.4%, compared with girls at 53.5%.
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 150ml 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. Make sure they have a glass of water before going to school and perhaps pack a bottle in their school bag, so they can take regular sips."
Dr Spungin said: "Plain water should be the first choice for all day long hydration."
Prof 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.
But 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.
He said parents and carers needed to be aware that children do not always pay attention to the feeling of thirst like adults do.
Prof 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.