A five-year-old girl was taken into care by social workers after her weight rose to more than 10 stones (67kg).
The child was at least three times the expected weight for her age.
She was taken into care in Newport, south Wales, in August last year and is understood to have been tested for a rare genetic condition which causes constant hunger.
Child protection experts have labelled the case 'a tragedy' and expressed disbelief that action was not taken sooner by the local council.
She weighed 10st 5lb when Newport City Council intervened - making her the heaviest boy or girl aged five recorded in an English school since 2008.
Typically girls of that age weigh about three stone, according to the Royal College of Paediatrics and Child Health.
Newport City Council told The Sunday Times it made the decision to take her into care purely because of the girl's obesity.
Two months later her weight had risen yet further to 10st 10lb, but when she was weighed again this September it had dropped to below eight stone.
However, her Body Mass Index – a measure of weight in comparison to height referred to as BMI – was still roughly double the normal level for a five-year-old child, expected to be around 3 stones, according to the Royal College of Paediatrics and Child Health..
According to the Body Mass Index scale on the NHS Choices website, 10 stone is a healthy weight for an adult if they are approximately 5ft 3in (1.7 metres) in height or above.
Speaking to The Sunday Times, Newport City Council said the decision to put the girl into care was based solely on her obesity but declined to say how she was allowed to reach such a high weight without prior intervention.
The data which revealed her case comes from a Freedom of Information request made by The Sunday Times in October. It also showed that 900 children, including 101 under five, had been admitted to hospital because of obesity in the past three years.
Tam Fry, honorary chairman of the Child Growth Foundation, said it was tragic the girl had not been taken into care earlier.
"Since that child was one year old she would have been putting on weight, year after year after year," he said. "She must have been visible at nursery. Who didn't raise their hand and say, 'Look, something is going tragically wrong here'?"
Newport council said: "The wellbeing of children and young people is of paramount importance and at the heart of all the work that is done by our children's services department.
"A thorough and comprehensive assessment of each child and family that we work with is always undertaken. All factors will be considered in any decision that is taken. It would be inappropriate to comment further on this case in the best interests of the child."