Parents should not expect schools to police children's eating and exercise, or toilet train pupils, Ofsted boss Amanda Spielman will say this week.

England's chief inspector for schools will argue the answer to the obesity crisis lies in the home, and parents should not "abdicate responsibility".

Neither can schools be a "panacea" for knife crime or child neglect, she will add in her second annual report.

Two studies have this year queried the benefit of school anti-obesity schemes.

In February, the British Medical Journal reported that a year-long anti-obesity programme involving more than 600 West Midlands primary school pupils yielded no improvements.

And in July an Ofsted study of 60 schools found no link between efforts to tackle obesity and pupils' weight.

Ms Spielman, who will present Ofsted's annual report on Tuesday, will highlight concerns that - by the time they start primary school - almost a quarter of children in England are overweight or obese.

This rises to over a third by the time they move on to secondary school.

"Schools can and should teach children about the importance of healthy eating and exercise in line with their core purpose; their PE lessons should get them out of breath," she will say.

"But beyond that, schools cannot take over the role of health professionals - and above all parents."

Highlighting the growing evidence of children arriving at reception unable to use a toilet, she will add: "This is difficult for teachers, disruptive for other children and has a terrible social impact on the children affected."

'Complex matters'

Ms Spielman will also argue that by expecting schools to tackle gang-related crime or child neglect, society risks not only distracting them from their core purpose but also failing to solve the problems.

Such complex matters need to be dealt with by those with the correct knowledge and expertise, she will argue.

"While schools can play a role in educating young people about the danger of knives, they cannot be a panacea for this particular societal ill," she will say.

"Instead, preventing knife crime requires all local safeguarding partners to work together to protect children from harm whilst the relevant agencies tackle criminal activity."