Tony Blair has owned up to smacking his three elder children, but said that he had never hit his five-year-old son Leo.
The Prime Minister disclosed he had changed his attitude to corporal punishment as he launched the Government's action plan for restoring a sense of respect to society, including extra help for parents.
Questioned by BBC2's Newsnight over whether he smacked his children, he said: "I'm probably different with my youngest than I was with my older ones."
The interviewer Kirsty Wark persisted: "What, you do smack the younger one?"
Mr Blair, taken by surprise, responded: "No-no, no-no. It was actually the other way round but ... I think, look, this smacking ... I mean, I agree with what you just said, I think everybody actually knows the difference between smacking a kid and abusing a child."
Children's groups said that the difference between his behaviour now and 20 years ago reflected a shift in attitudes to corporal punishment over the past two decades.
Under the Children Act, which came into force last January, parents in England and Wales who hit children so hard it leaves a mark will face up to five years in jail.
Mild smacking is defined as "reasonable chastisement" and is allowed under the legislation.
But any punishment which causes visible bruising, grazes, scratches, minor swellings or cuts can lead to prosecution.
Previously Mr Blair has backed parents' right to use corporal punishment to discipline their children.
Children's charities, which are campaigning for a total ban on smacking, welcomed the Prime Minister's apparent change of heart. Caroline Abrahams, director of policy at NCH, said: "It reflects something that is happening more generally in society.
"Twenty years ago the idea that someone shouldn't smack children was much more contentious than it is now. We are against it in all circumstances, although we recognise parents sometimes lose their temper and do something they subsequently regret."
An NSPCC spokesman said: "Of course parenting can sometimes be hard work, but there are better and safer ways to discipline your children than physical punishment - such as praising the behaviour you want to encourage, while ignoring the smaller problems that children soon grow out of."
Nine years ago Mr Blair told how he had resorted to disciplining his eldest three children. He said: "When they were little I smacked them occasionally if they were really naughty or did something nasty to another child." He added: "I always regretted it because there are lots of ways of disciplining a child and I don't believe that belting them is the best one."
But he said: "There is a clear dividing line between administering discipline on the one hand and violence on the other, which most parents understand perfectly well.
"The important point is to discipline your children, because they must realise that there are some things they can't do."Reuse content