Councils in Wales are unprepared and “in the dark” over the implications of the proposed smacking ban, critics have claimed.
The Welsh Government published a bill removing reasonable punishment as a defence in March this year.
If passed by the Assembly, it will be made law and give children the same protection from physical punishment as adults.
It would mean that if accused of assault or battery of a child , parents and guardians could not use the current defence open of reasonable chastisement
Supporters say smacking children is unacceptable and the planned new measures would protect kids from harm.
But opponents argue changing to the law is unnecessary and would criminalise parents.
Now, campaign group Be Reasonable said county councils - already over stretched in terms of finances and resources - have not had clear guidance from Welsh Government on the changes, which could cost millions of pounds.
It submitted Freedom of Information (FOI) requests to Wales' local authorities - and said most seemed confused about how the new law could affect them.
When asked about the potential impact of banning smacking on its social services, Flintshire Council responded: “To date we have received no guidance or policy documents from the Welsh Government.”
And Bridgend Council said it did not have any idea about the number of additional families it would have to engage with, or the number of extra social workers that would be needed following the introduction of a ban.
Campaign spokesman Jamie Gillies said: “These are worrying admissions. It seems that councils have been left in the dark over the impact of these changes and have received no information or advice from the Welsh Government about how they should prepare for a smacking ban.
"Council budgets are already stretched. Any change in the law which places an additional burden on local authorities could lead to tough financial decisions and vital services being pared back. Councillors and the public alike deserve to know how their community will be impacted.”
A Welsh Government spokesman said it could take as long as two years for the law to change.
But Be Reasonable insisted it was not too early for councils to look at possible cost implications given that they must plan ahead and set their budgets for the coming year in the next few months.
A report from the Assembly’s Children Young People and Education Committee earlier this month estimated the Bill would cost between £2.3m and £3.7m, including a public awareness campaign.
It added that the Welsh Government estimates between £0.97m and £0.98m will be needed for for police and justice services.
Be Reasonable predicted this would have knock on effects to councils too.
"Many councils admitted that they don’t know how this policy will affect social services departments. This is unsurprising as the Government itself has failed to give any indication of the impact,” said Mr Gillies.
"The Government predicts that hundreds of families (548 per year), will be caught out by this change.
"It can’t be right that this legislation is moving forward with no estimate whatsoever about how children’s services will be impacted. The Children’s Committee recently asked for more clarity on this question and the Government must provide this soon."
So far 15 of Wales’ 22 councils have responded to FOIs sent out by Be Reasonable, which has around 10,000 supporters in Wales.
Merthyr Council also said it had made no cost assessment for any change to the law, but Neath Port Talbot Council said: "Steps have been taken to collate the numbers of contacts and referrals within the Social Services Intake Team from July 2019 in anticipation of the change in the law."
A Welsh Government spokesman said: “We want to make it clear that physically punishing a child is no longer acceptable in Wales. Proven positive alternatives are much better for our children and their wellbeing.
"We acknowledge a parent who physically punishes their child could be charged with a criminal offence after the law changes; this is why we will be undertaking a significant campaign to raise awareness of the law change and inform parents about alternatives to physical punishment.
"Representatives of local authorities and children’s services have given evidence to say they support this Bill. They are also taking part in developing guidance on implementing the Bill and we will allow up to two years to plan for the law to come into force. Social workers and other professionals working with children say the Bill will help them by providing clarity on the law.
"There is a growing consensus internationally that the physical punishment of children should be prohibited by law. Other countries have already legislated for this.
"We have already outlined the possible impact on the social care system. While there may be an initial increase in reporting of incidents we do not expect any additional children will come under the remit of the care system as a direct consequence of this Bill."
