Smacking of children is an emotive issue

In a letter to Education Secretary Ruth Kelly, they say rules should be changed as a matter of "fundamental principle".

Legislation which was tightened last year allows parents to hit their child mildly as a "reasonable punishment".

Prime Minister Tony Blair has admitted smacking his older children. Other campaigners say smacking should not be confused with child abuse.

The United Nations and the council of Europe have both made it very clear that all ... assaults on children must be made illegal Kathleen Marshall

Kathleen Marshall, Scotland's Commissioner for Children and Young People, told the BBC that the UK was failing to meet its obligations.

"We must insist on a total ban because this isn't an issue that's going to go away," she said.

"The smacking of children, the hitting of children in what in Scotland is called 'justifiable assault' is prohibited by international law.

"The United Nations and the council of Europe have both made it very clear that all physical punishment, all assaults on children, must be made illegal, and so we would be failing in our duty as children's commissioners if we did not say that we have to keep this on the agenda."

'Smacking is hitting'

The Department for Education and Skills responded in a statement: "We do not condone physical punishment for children but it is for parents to decide.

"They have to think carefully about the law on assault and make sure that chastisement does not get to that point. If not they will be prosecuted."

The Children's Right's Alliance told BBC Radio 5 Live's Stephen Nolan Show that any form of physical punishment is wrong.

Spokeswoman Caroline Willow said: "If you look at it from adults we have special words like slap and tap and smack, which disguise the violence that's being inflicted on children.

"If you ask children themselves, which I've done, children are very clear that smacking is hitting, and that it hurts and that parents regret it and there's better ways to deal with situations."

The BBC's Laura Bundock said smacking is a controversial issue which came to the fore once again with the prime minister's recent admission.

'Not violence or abuse'

She said the commissioners' statement proposes "children are given the same protection from assault as an adult".

"They argue smacking confuses parents, inhibits child protection and undermines the promotion of positive forms of discipline."

She added: "With increasing bans in European countries, the UK law could well face similar changes."

However, Mary Russell, from the Family Education Trust, told the BBC it was wrong to confuse the imposition of discipline with violence inflicted upon children.

"We're not talking about abuse, we're not talking about violence, we are talking about a certain smack or a tap or whatever.

UN rebuke

"I'm not talking about an assault and I don't think anyone would want to muddle up some sort of assault on a child with normal parental loving discipline."

"Reasonable chastisement" is a defence against common assault but any punishment which causes visible bruising, grazes, scratches, minor swellings or cuts can result in legal action.

In 2002, the United Nations' Committee on the Rights of the Child said it "deeply regrets that the (UK) persists in retaining the defence of 'reasonable chastisement'.

In 2003, the Scottish Parliament voted to restrict the scope of "reasonable chastisement" by setting out criteria for determining whether an assault on a child was "justifiable".

One year ago MPs voted to restrict "reasonable punishment" in England and Wales.