Tony Blair was under growing pressure last night from inside the Labour Party to ban smacking as MPs and peers claimed the current rules allowed parents to abuse their children.

A vote in the House of Lords expected this week will see a cross-party alliance of more than 100 peers call for a ban, other than in exceptional cases such as when a child is in danger or might hurt another child.

The Government has so far resisted changing the law, fearing it would lead to thousands of cases in which parents would be prosecuted for minor offences.

Labour strategists have maintained that the party would be seen as interfering in the relationship between parent and child and running a "nanny state".

Labour has also refused to grant a free vote in Parliament on the issue.

But last night there were indications that ministers might compromise rather than suffer an embarrassing defeat on the issue.

Government insiders indicated that they might be prepared to see a virtual ban imposed so long as this protected parents from prosecution in cases where they merely clipped a child or gave some other form of gentle physical reprimand.

Under a definition of "reasonable chastisement", which dates back to the mid-nineteenth century, parents can currently use a degree of force to discipline their children.

This often affords them protection in cases involving abuse.

Now more than 200 peers and MPs, including many from the Labour side, have signed up to a campaign to amend the current Children's Bill passing through Parliament.

The Bill puts into effect recommendations from the inquiry into the death of Victoria Climbie, who died after repeated abuse.

David Hinchliffe, the Labour MP who chairs the House of Commons select committee on health and used to work in child protection before becoming an MP, said he was planning to table his own amendment when the Bill returned to the House of Commons.

Mr Hinchliffe said: "The reason I feel so strongly about this is that I have seen terrible cases of what has happened to children as a result of abuse but the social services have lost the case because of this law.

"It is out of date and needs changing.

"The child protection agencies are working with one arm tied behind their back at the moment."

The Tories said, however, that the Government should stand firm against a ban on smacking.

Theresa May, the shadow secretary of state for the family, said it was not up to ministers to spend all their time "trying to tell people how to run their lives".

"We all know there is a limit beyond which parents should not go and at the point where what is reasonable control actually becomes abuse, that obviously is wrong and Government and society has a right to step in at that point," she said.

Conservative peers are expected to be offered a free vote in the Lords.

Sir William Utting, from the "Children are Unbeatable!" campaign, said the current law was archaic and condoned physical abuse against children that would be criminal if it were carried out on adults.

Sir William added: "Hitting children is as unacceptable as hitting anyone else and the law should clearly say so."