AMs vote on introducing a ban in the Senedd next week.
The issue has caused tensions within the Labour group, with the Welsh government ruling out changing the law before next year's assembly elections.
One Labour AM, Christine Chapman, spoke last year of feeling "cheated" that ministers had not addressed the issue.
In a joint letter published in this morning's Western Mail, the Most Reverend Barry Morgan, Professor Sally Holland and Phil Bale, leader of Cardiff Council, argue that a ban would simply give children the same protection in law as adults.
They write: "We call on the First Minister and Welsh Government to allow Labour AMs a free (conscience) vote on an amendment to the Violence against Women, Domestic Abuse and Sexual Violence (Wales) Bill concerning the physical punishment of children, and we call upon all AMs to support this amendment.
"The amendment would remove the defence of 'reasonable punishment' in relation to assaults on children in Wales and finally give children the legal protection from physical harm that they deserve."
Labour's current position is that there will not be a free vote for its AMs on the amendment, tabled by Cardiff North AM Julie Morgan.

Hull succeeds Derry/Londonderry as the UK's City of Culture, after beating Leicester, Dundee and Swansea Bay for the rights to the 2017 title.
Officials have described winning the title as a "real game-changer", and the "beginning of a cultural renaissance".
Hull council leader Stephen Brady said: "It will give Hull a platform to tell the world what this great city has to offer, transform perceptions and accelerate our journey to make Hull a prime visitor destination."
Abigail McIntosh, who went to university in Hull, published photos of the city on Buzzfeed in response to the negative image the city sometimes attracts.
It is jokingly titled 29 Things That Prove You Should Never, Ever Go To Hull.
She said: "When I applied to Hull, and received an offer, I was apprehensive because I had only ever really seen bad things about Hull in the press."
Ms McIntosh said she hoped events would help change people's attitudes.
The first event of the year will be an installation across the city, telling the story of the last 70 years of Hull, accompanied by a fireworks display to "rival London's New Year's Eve celebrations", officials said.
A "musical Humber Bridge", an art work featuring thousands of naked, painted people on the streets of Hull are part of the £32m programme taking place in 2017.
Made In Hull will be free and will run every night from 1-7 January.
Deborah Stevenson, who regularly posts about the city on social media, said:  "I think Hull needed this, it was down and out and at the end of the line.
"We have the history of the city, but to be able to build the tourism we need to celebrate it more."
In the run-up to 2017, locals have faced traffic misery while improvement work takes place.
People have been asked to open up their houses to accommodate visitors, due to a shortage of hotel beds.
But Mr Brady said what was planned would "improve the lives and opportunities of everyone who lives, works, visits and invests here".
"For Hull, 2017 is just the beginning," he said.
2013 was a year Derry will never forget. From BBC Radio 1's Big Weekend to the Turner Prize, the eyes of the world were on Derry and it certainly felt there was a new confidence to the city and its people.
Three years on though, our listeners have have been left asking what do we really have to show for it? Legacy funding dried up in 2015, with city of culture community projects, which were such a success, suffering.
The advice to Hull would be enjoy every second of your title year, but keep in mind what you want to have to show for it once the party is over.
The Archbishop of Wales and the new Children's Commissioner have led calls for AMs to be given a free vote on a ban on smacking children.
[NEXT_CONCEPT]
As the schedule of events for Hull's year as City of Culture are announced, BBC News looks at what the title means for the city, and what is has to offer.