According to leaked extracts from the Macpherson report into the Stephen Lawrence murder investigation, the inquiry is to recommend an end to the law that says a person cannot be charged twice for a crime.

The law is sometimes known as the rule against double jeopardy.

For centuries the courts have considered it unfair for people to be harassed by repeated prosecutions for one crime. Justice is an important principle, but the judges also insist on certainty and finality.

Joshua Rozenberg: "Civil liberties groups will be concerned about any change to the law"

The precise ambit of the double jeopardy rule has recently been refined by the courts but there is now a major exception to it.

Under the Criminal Procedure and Investigations Act of 1996 it is possible for a defendant to be tried again if his acquittal was tainted because someone has intimidated or interfered with a witness or a jury member.

This law applies only to offences committed after April 1997, so it has no direct relevance to the murder of Stephen Lawrence, which took place in 1993.

But it does show that Parliament is willing to make changes to long-established legal principles.