Sexual infidelity could be a defense in murder cases as it can cause people to lose control, Britain's most senior judge has ruled.
Lord Judge, the Lord Chief Justice, said juries should be allowed to consider the fact a victim had been unfaithful as a possible provocation - in defiance of a new law that banned it as an excuse.
It restores a defence in so-called "crimes of passion" but Lord Judge insisted infidelity cannot be the sole reason for the murder and other "triggers" must be shown, the Telegraph reported.
The last Labour Government changed the law surrounding murder defences to exclude sexual infidelity in "loss of control" killings.
But Lord Judge said to exclude it in cases where it was integral was to risk "injustice" and that the law was likely to "produce surprising results".
His ruling risks a fresh conflict between the judiciary and MPs after effectively dismissing a law drawn up by parliament.
When the law was first drawn up in 2009, it was met with fierce opposition in the House of Lords, where one peer described the move as "outstandingly obnoxious".
The Coroners and Justice Act 2009, which came in to effect in 2010, replaced the murder defence of provocation with a new partial defence of "loss of control", but exempted sexual infidelity as a reason for such a loss.
However, following the Appeal Court ruling, those accused of murder can once again use evidence that their victim was unfaithful to them as part of the reason why they snapped and killed them.
Lord Judge, sitting with two other judges, gave the general guidance after hearing three "loss of control" test cases.