A five-year-old girl weighing over 10 stone (63kg) was taken into care by social workers last summer in what has been called a tragic case.
The girl weighed three times more than an average five-year old child, according to the Royal College of Paediatrics and Child Health.
Experts in child protection have called the case a "tragedy" and could not understand how the local authority or nursery staff had not intervened sooner.
She was taken into care in August last year in Newport, South Wales, at which point she weighed 10st 5lb: heavier than any five-year-old girl or boy recorded in a UK school since 2008.
Two months later she had gained a further 5lbs, but her weight dropped below eight stone by September 2013, still over double the 3 stone a child of her age should weigh.
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 5' 3" (1.7 metres) in height or above.
According to a report published by the Welsh Assembly in July, 35% of children under the age of 16 are classified as overweight in Wales, making the nation's rate of childhood obesity the highest in the UK.
Diseases including type II diabetes, heart disease and cancer were flagged up as some of the consequences of obesity in the report. Severely obese people, it added, were estimated to die 8-10 years earlier.
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.
Gwent Police could not confirm whether anyone had been arrested in connection with the case without more information from the local council.
The data which revealed her case comes from an FOI 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'?"
Child protection consultant Joanna Nicolas in her 18-year career as a social worker she had never seen a five-year-old girl weigh 10 stone.
"Why didn't somebody do something sooner rather than picking it up at this stage? Professionals would have tried to do things along the way, but action should have been taken a lot sooner," Nicolas said.
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."