The children's secretary wants an urgent decision on whether the law should be changed to close a loophole that allows children to be smacked by Sunday school teachers and private tutors

The children's secretary, Ed Balls, has ordered an urgent inquiry into whether Sunday school teachers and private tutors should be allowed to smack their pupils.

A loophole in the law means that while teachers in state and private schools are banned from smacking children, their counterparts in faith schools are not.

Teachers who take pupils for fewer than 12.5 hours of lessons a week have the same status as someone who is standing in for a parent, and can therefore give a child a mild smack. They can plead the defence of "reasonable punishment".

Balls has demanded that the government's chief adviser on children's safety, Sir Roger Singleton, report to him within a week on whether the law should be changed.

The issue has been raised by Ann Cryer, a Labour MP, who wants the loophole closed.

In a letter to Singleton, Balls wrote that the government would like to "progress to a point where smacking is seen as unacceptable by the vast majority of parents, and is only used as a last resort, if at all".

But ministers would stop short of making smacking illegal because it would "criminalise decent parents who decide to administer a mild smack," he said.

Balls wrote: "We recognise that whilst it seems that fewer parents smack their children, most currently do not believe they should be banned from doing so by law. Our approach is to provide parents with support and guidance to help them manage their children's behaviour more effectively.

"The defence of reasonable punishment may be available to those who teach in certain part-time educational and learning settings, for example religious instruction that children attend at the weekend. I am concerned to establish the key issues here and whether this is an area in which we need to consider a change, in the interests of strengthening safeguards for children."

But the schools minister, Vernon Coaker, said he feared a change in the law could create "unintended problems" such as stopping fathers from smacking children they care for, but for whom they do not have parental responsibility.

In his reply to Cryer in the House of Commons last week, Balls said: "The important point to make is that there is not one rule for a child in a madrassa and another for a child in any other circumstance.

"The use of physical punishment against any child is wrong; it is outside the law and is not fair to children.

"I do not think we should tolerate any use of physical punishment in any school or learning setting in which trusted adults are supposed to be looking after children, not abusing them."

David Laws, the Liberal Democrat schools spokesman, said: "The government needs to legislate to protect children – not leave an opt-out simply because it fears some ethnic or religious backlash."