New smacking laws may prove unworkable as doctors will not want, or will be unable, to give evidence when parents are prosecuted, an expert has warned.
Doctors are wary of giving evidence in child protection cases
The measures, due to come into force early next year, allow mild smacking but outlaw hitting that leaves a mark.
Professor Sarah Stewart-Brown, from Warwick Medical School, said the law for England and Wales was too ambiguous
for doctors to offer judgements on.
Government officials defended the ban as a "common-sense decision".
The law was agreed by the House of Commons earlier this month as part of a compromise amendment to the Children Bill.
Some MPs had called for an outright ban on smacking.
Prof Stewart-Brown, a professor of public health at the school, said MPs had made a "poor decision".
Writing in the British Medical Journal, she argued health professionals were going to have to deal with the consequences of the legislation as it would be up to paediatricians, and perhaps nurses and GPs, to adjudicate on whether the smack had left a mark.
She said there was already a shortage of doctors to give evidence in child protection cases as a result of the increasing number of complaints about colleagues involved in such work and recent cases where experts' evidence had been disputed.
Prof Stewart-Brown told BBC News: "The law is pretty ambiguous, doctors are not going to be able determine what constitutes a smack that is too hard.
"You also have to remember child protection work is very demanding - doctors are increasingly saying they do not want to do it."
She said an outright smacking ban should be introduced coupled with parental education support, as severe smacking can cause mental and physical damage.
"Harsh physical punishment, particularly when it is inconsistent, is a cause of criminality and violence with all its associated health problems," Prof Stewart-Brown said.
"It is time children were given the same protection as adults."
She was backed by the Royal College of Paediatrics and Child Health.
A spokesman said: "We have reservations about the Children Bill.
"What constitutes an offence? If we are going to use reddening, then different people react differently to smacks. It is going to be extremely hard for doctors to give evidence.
"And anecdotal evidence suggests more and more doctors do not want to give evidence in child protection cases."
Lucy Thorpe, a policy adviser for the National Society for the Prevention of Cruelty to Children (NSPCC), said the ban would create "legal ambiguity, widespread confusion for parents and professional uncertainty for those working with children and families".
She added: "The only just and safe reform, which would also fulfil our human rights obligations, is to give children equal protection under the law on assault, rather than persist in giving the most vulnerable the least protection."
A Department for Education and Skills spokeswoman defended the ban, saying: "This common sense decision balances the essential need for children to be protected with the right of parents not to have the government interfering in family life.
"The decisions professionals take regarding whether something should be considered assault, ABH, or GBH are the same decisions that they've been taking for many years. This has not changed."