Prime Minister Tony Blair today said he understood why parents would want to smack "really naughty" children.

But he said he felt "a bit different now", referring to his son Leo, aged four, and called for a "dose of common sense" in the debate.

His comments to BBC Radio 2 came as campaigners for a ban on smacking children were vowing to fight on after the Lords last night agreed parents should be able to use "mild" punishment.

Peers last night voted for a compromise measure outlawing smacking that causes lasting harm over an outright ban.

The decision was denounced as "shameful, unjust and irresponsible" as MPs prepared for a second push in the Commons.

The Lords last night voted to make it a criminal offence to cause bruises or mental harm in England and Wales.

The vote came as peers discussed the Children Act, aimed at increasing protection for youngsters in the wake of the Victoria Climbie case.

Mr Blair told Radio 2, in a pre-recorded interview broadcast this afternoon: ''Whenever you get into this argument, I think it's really, really difficult.

''I know, as a parent, if your child is really naughty you will want to smack them.''

Asked if he smacked his children, Mr Blair replied: ''With the little one I feel a bit different now. ''A dose of common sense is what you need in any of these arguments.''.