Parents could be prosecuted for causing harm to their child. Proposals to limit the right of parents to smack their children will be debated in the House of Lords later on Monday.
A Lib Dem's amendment to the Children Bill seeks to outlaw smacking if it causes physical harm, such as bruises, scratches or reddening of the skin.
The government has given Labour peers a free vote on the change - but has told them they must vote against another amendment backing a total ban.
Novelist Salman Rushdie, who wants a complete ban, called it a "fudge".
Peers will vote first on Monday afternoon on the complete ban - and only vote on Lord Lester's compromise amendment if it is rejected.
Ministers have been accused of reneging on a pledge to outlaw smacking by telling peers to oppose a complete ban, and by giving them a free vote on Lib Dem Lord Lester's amendment.
Flood of prosecutions
His amendment allows "moderate smacking" but removes the defence of "reasonable chastisement".
Parents who inflict bruising or reddening of the skin, or cause mental harm could be prosecuted.
" If [the government] had played fair and given peers a free conscience vote on the equal protection clause, they would not be scrabbling around now tacitly supporting proposals which mean inequality for children."
Children Are Unbeatable
Ministers fear an outright ban would lead to a flood of prosecutions over minor offences.
Health Secretary Dr John Reid said he believed most people would back the government's approach, which will take shape during a House of Lords debate on the Children Bill on Monday.
But Mr Rushdie, who supports Children are Unbeatable, an umbrella group for more than 350 organisations campaigning for a complete ban, said the government had pledged a ban six years ago.
"Our view is quite simple - there is a human rights issue and this is not a question of nannying it's a question of increasing personal freedom ... that's to say to give children the personal freedom not to be hit," he told BBC Radio 4's Today programme.
Conservative spokesman Theresa May said her party did not have an official "position" on smacking, preferring to leave it up to individuals' conscience. She said she personally did not support a total ban.
The official Lib Dem policy is to give children equal legal protection from assault as adults.
The party's children's spokeswoman in the Lords, Baroness Walmsley, is backing a complete ban on smacking.
She said: "Assaulting a child is as unacceptable as assaulting an adult, and the law should clearly say so."
Actual bodily harm
Under a definition of "reasonable chastisement", dating back to 1860, parents can currently use a degree of force to discipline their children.
Under Lord Lester's amendment, any parent who inflicted actual bodily harm on a child could be prosecuted and would no longer have the protection of the defence.
Supporters of removing the "reasonable chastisement" defence say it is often used by parents as their defence in cases involving abuse.
More than 200 peers and MPs, including many from the Labour side, signed up to a campaign to amend the current Children's Bill passing through Parliament.
The Bill puts into effect recommendations from the inquiry into the death of Victoria Climbie, who died after repeated abuse.
Labour MP David Hinchliffe, who chairs the Commons select committee on health and used to work in child protection, has said the child protection agencies "are working with one arm tied behind their back" with the "outdated" current law.
HAVE YOU BEEN
TO THE NEWSROOM?