The murder of model Rachel Nickell, who was stabbed more than 40 times in front of her two-year-old son, remains unsolved after the case against suspect Colin Stagg was thrown out of court.

Janette Clucas reports: "1 in 10 of such cases are outstanding"

An Old Bailey judge ruled that evidence obtained by an undercover policewoman against Mr Stagg was unadmissable.

The officer, known only as Lizzie, encouraged Mr Stagg to swap violent sexual fantasies in an attempt to lure him into making a confession. The judge Mr Justice Ognall refused to even allow the case to be put before a jury and returned a verdict of not guilty.

Stagg: Acquitted of murder

The murder of Ms Nickell is to be re-examined as part of a new policy review of all unsolved killings in London.

Detectives from the Metropolitan Police will be hoping to uncover fresh evidence using new DNA techniques.

They will also re-examine all witness statements made after the south London murder.

The move is partly in response to a recommendation in the Stephen Lawrence report.

All unsolved murders in the London area will now be reviewed every month while old unsolved cases will be reviewed every two years for possible breakthroughs.

Earlier this week police announced they are to review key evidence in the case of PC Keith Blakelock, who was murdered in a London riot 14 years ago.

The officer was killed during the Broadwater Farm riot in Tottenham, north London, in October 1985. His killers have never been brought to justice.

In 1987 three men - Winston Silcott, Mark Braithwaite and Engin Raghip - were convicted of PC Blakelock's murder, but the convictions were quashed on appeal.

Silcott, who is currently serving life for another murder, is suing the Metropolitan Police for malicious prosecution.

The case will be heard at the High Court in June 2000.