Women who kill abusive partners in cold blood could escape a murder conviction if they prove they feared more violence.
Under a major government review, they will be punished for the lesser offence of manslaughter, sparing them a mandatory life sentence.
They must establish only that they were responding to a 'slow burn' of abuse.
The change sweeps aside the existing requirement in any defence of provocation that they killed on the spur of the moment after a 'sudden' loss of control.
In cases where a husband kills, the existing 'partial defence' of provocation if a wife was having an affair is scrapped altogether.
The Ministry of Justice said this was in response to long-standing concerns that the centuries- old measure impacts differently on men and women.
In the first major changes to homicide laws in 50 years, ministers have ruled that other categories of killer, as well as domestic violence victims, should be offered new partial defences of provocation.
They include those 'seriously wronged' by an insult.
Beneficiaries of this change may include those who strike out after long and bitter disputes with neighbours, or victims of a serious crime who are taunted at a later date by the attacker.
Instead of receiving a mandatory life sentence for murder, they too could escape with a manslaughter conviction.
Women's groups had long campaigned for changes to the law to protect victims of domestic violence who hit back in desperation.
But the proposed new partial defence for killers who feel 'seriously wronged' by 'words and conduct' took experts completely by surprise.
Robert Whelan of the Civitas think-tank accused Ministers of introducing 'gang law' into the legal system.
He said: 'To take someone's life because they say something that offends you is the law of gang culture.
'Are we really going to introduce into our criminal justice system that it is a defence to say "I was insulted"?'
He also voiced concern about the plan to give special protection to certain groups.
Mr Whelan said: 'By creating all these special categories, the Government are making some people more equal than others before the law.
'It seems some lives are worth more than others.'
Lyn Costello of Mothers Against Murder and Aggression described the changes as 'utter madness'.
She warned: 'We need clear laws, not more grey areas. This is not the sort of message to send out.
'You will have some very clever lawyers who will twist this around to suit their clients.
'Unless there are really exceptional circumstances, such as self defence or protecting yourself or family, then there is no excuse for killing someone and it should be murder.'
Officials, however, denied they were creating any loopholes.
In an interview with the Daily Mail, Justice Minister Maria Eagle gave an example of where the new defence could apply as a 'serious neighbour dispute' in which the provocation of one person had reached a 'very high level'.
She also cited a person who had been subjected to repeated racist abuse.
Her officials stressed later that any neighbour dispute would have to go 'quite beyond what an ordinary person should be expected to deal with'.
Other examples of where the defence may apply included a victim of a serious crime, such as a rape, being taunted by their attacker at a later date, or a mother who came home to find a man trying to rape her daughter, chased him down the street and stabbed him in the back.
The new defence reads: 'In exceptional circumstances only, killing in response to words and conduct which caused the defendant to have a justifiable sense of being seriously wronged'.
The defences of both fear of serious violence and being 'seriously wronged' apply only in cases where a person is deemed to have 'lost control', rather than acted in a premeditated manner.
This is itself a weakening of the current law, which specifies that a person using such a defence must have have suffered a 'sudden' loss of control.
The 'seriously wronged' clause also makes a special exemption for infidelity.
Ministers said sexual jealousy could no longer be used as a defence under any circumstances.
Harriet Harman, Labour's deputy leader and Minister for Women, said: 'After a man has intentionally killed his wife, bereaved relatives have said to me "Why is he allowed to try and get away with murder?
'"He planned to kill her, he intended to kill her, he did kill her. How is this not murder?"
'At the moment the law allows him to try to get off a murder charge by claiming she provoked him, for example by being unfaithful.
'It's unacceptable if you've lost a sister, or a mother, to then be told it's her fault because she provoked him.
'Changing the law will end this injustice of women being killed by their husbands and the injustice of them then being blamed.
'And it will end the injustice of the perpetrators making excuses saying it's not their fault.'
The proposals will be part of a Bill to be included in the Queen's Speech in November.
They will then go through Parliament over the following months.
Justice Department officials said that, overall, they expected the number of murder convictions to increase by 20 each year as a result of the whole package of changes.
Ministers have, however, decided not to adopt two proposals suggested by the Law Commission, the Government's law reform advisers.
They are for a U.S.-style system of first and second degree murders, and a recommendation for a defence of 'developmental immaturity'.
This would have seen children who kill being convicted of lesser charges if their lawyers could prove that they were young for their age.