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(UK) The first comprehensive review of the law of murder for more than 50 years will recommend a fundamental change that would see many killings downgraded to manslaughter, the Guardian has learned.

Under the proposal, only homicides where the killer intended to kill will be classified as murder. At present, murder also includes cases where the killer intended to cause serious bodily harm. Restricting murder to cases where death was intended would remove a large number of cases where the victim dies as a result of a fight or an attack during a robbery or burglary which proves fatal.

Changing the definition of murder is the key recommendation to be published tomorrow in a report by the Law Commission, the official body drafted in by the Home Office to assist in the review of the laws governing the crime.

According to one QC, if the change is accepted "you will remove from the category of murder a very, very large proportion of the cases which currently are found to be cases of murder". In one House of Lords case it was suggested that fewer than half of convicted murderers were convicted on the basis that they had intended to kill.

The reform the commission recommends would mean that only the most serious killings would attract a mandatory life sentence. For those where death was not intended, judges could still sentence the killer to life but would have a discretion to impose a lesser sentence.

The recommendation is the main plank of the first stage of a review announced in October 2004 by David Blunkett, then home secretary. His announcement was a response to a Law Commission report on provocation and diminished responsibility in August 2004, which said the law on murder was "a mess" and needed to be looked at as a whole. Many judges and academics believe much of the mess would disappear if the mandatory life sentence were scrapped, leaving it to the judge to sentence according to the circumstances of the offender and the crime.

The commission wanted that as an option but the Home Office insisted in its terms of reference for the review that the automatic life sentence should remain for murder. The Home Office review team will now consider the wider public policy issues and take into account the views of the public. The ultimate aim is new legislation to replace the Homicide Act 1957, widely regarded as outdated.

Ken Macdonald, the director of public prosecutions, has complained that prosecutors currently had to choose between charging murder or manslaughter and ran a greater risk of losing the case and allowing a guilty defendant to go free if they opted for the more serious charge.

Under the new definition, many recent high-profile cases may not have been prosecuted as murders, including the death of Damilola Taylor. In that trial, the youths who were accused of attacking the 10-year-old with a knife were charged with murder. They were all cleared.

The Law Commission report will also recommend simplifying and clarifying the current "partial defences" of provocation and diminished responsibility which reduce a charge of murder to manslaughter. Judges have called for urgent reform, arguing that the state of the law risks miscarriages of justice because it is too complicated for juries to apply. The report is not expected to recommend putting mercy killing in a separate, lesser category of homicide. Although mercy killing is murder, juries are reluctant to convict and defendants usually successfully plead diminished responsibility, reducing the offence to manslaughter and allowing the judge to pass a non-custodial sentence.