Campaigners opposed to smacking children have vowed to fight on after peers rejected an outright ban.
Parents could be prosecuted if they cause 'actual bodily harm'
On Monday the Lords voted for a compromise measure that only bans parents from hitting their children if it causes lasting harm.
The government opposes a ban and had told Labour peers not to back one.
But senior Labour MP David Hinchliffe said 100 backbenchers and a "significant" number of ministers wanted smacking outlawed altogether.
The health committee chairman called on the government to allow MPs a free vote on the issue.
Mr Hinchliffe said: "I genuinely hope the government will have the commonsense to allow a free vote and not whip it.
"If this is not an issue of conscience, I don't know what is."
But on Monday Tony Blair's spokesman explained the government's opposition to a smacking ban.
"We do not want to criminalise parents," he said.
The compromise measure backed by peers was put forward by Lord Lester by 226 votes to 91.
The Lib Dem peer's amendment to the Children Act removes the defence of "reasonable chastisement" which was introduced in 1860.
Smacking which causes bruising or mental harm to children is liable to end in prosecution.
Tony Samphier, of the Children Are Unbeatable alliance, said peers had "failed children".
"It sends out a dangerous message to society that it is still legally acceptable to assault a child," he said.
"Hitting children is as unacceptable as hitting anyone else, and the law should clearly say so.
"Equal protection from assault for children is the only responsible and safe way to modernise the law."
Earlier Labour peer Lord Peston called the government's decision not to grant a free vote on the total ban, rejected 250 to 75, "shameful".
Tory spokesman for the family, Theresa May, said there was no point to legislation which may prove to be
"Who will decide what constitutes an acceptable smack and one which breaks
the law?" she said.
"We do not want to see parents, often bringing children up in difficult
circumstances - made into criminals for doing what they believe to be in the
best interests of their families."
Lord Lester's amendment was backed by Lord Laming, who led the inquiry into the death of Victoria Climbie, an eight-year-old who died from the abuses and tortures inflicted by her great aunt and her partner.
Culture of violence?
"We would all like every parent to have the skills and ability not to have to resort to physical punishment," he told the BBC.
"The reality is quite different at the present time for many parents who may be tired, may be exhausted, may be harassed for a whole variety of reasons," he added.
"The most important thing is to pursue constructive approaches towards the family and not rather negative ones."
During the three-hour debate leading to the vote in the House of Lords, Liberal Democrat Baroness Walmsley begged peers to vote in favour of the full ban.
She said Britain's "culture of violence" was rooted in the acceptance that children could be smacked.