Men and women facing domestic violence could argue they were forced to kill their tormentor under a new murder law which comes into force today.
Killers can escape a murder conviction by proving they were motivated by "words and conduct" which left them "seriously wronged".
Under the changes, the defence of provocation is replaced with a new defence of "loss of control" caused by "a fear of serious violence" or in response to "words or conduct which caused the defendant to have a justifiable sense of being seriously wronged".
A Ministry of Justice spokesman said: "Changes to the law on murder contained in the Coroners and Justice Act 2009 come into effect today.
"These changes are based on Law Commission recommendations made in their review of homicide law in 2006 and were fully debated by Parliament and passed into law in 2009.
"These changes will strengthen the law and provide for more just and equitable outcomes in individual cases."
The new law also replaces the partial defence of diminished responsibility with a new defence based on "recognised medical conditions".
The Infanticide Act 1938 is amended to make clear that the offence and defence of infanticide are only available in respect of a woman who would otherwise be found guilty of murder or manslaughter.
Men and women who kill after suffering a "slow burn" of domestic violence over a period of time could use one of the partial defences under the new law, which replaces a requirement for them to have acted on the spur of the moment.
The old law made it too easy for men to kill their wives and claim they were provoked by the victim's infidelity, but at the same time restricted the use of partial defences by women with abusive partners.
The proposals for a "slow burn" defence, where a killer takes a life after being subjected to delayed or gradual pressure, would still have a high threshold and apply equally to both men and women.
The "fear of serious violence" defence could apply, for example, when a mother kills a man after catching him trying to rape her daughter, a Ministry of Justice spokeswoman said.
Women's Aid, which works to end domestic violence against women and children, welcomed the changes.
Its chief executive Nicola Harwin said: "The new revised partial defence of provocation should help deliver fairer treatment of domestic violence victims in cases where they have killed a violent and abusive partner, often following years of abuse.
"We also welcome the fact that infidelity will no longer be treated as an acceptable defence for anyone killing a current or former partner."