A convicted killer and sex attacker pleaded guilty today to stabbing Rachel Nickell to death in a frenzied attack on Wimbledon Common 16 years ago.

Robert Napper's plea at the Old Bailey means that Colin Stagg - for years the police's prime suspect for the murder - has finally been exonerated. The Metropolitan Police was to make a formal public apology to Mr Stagg today, bringing an end to a particularly bleak episode for Scotland Yard.

Napper, 42, pleaded guilty to manslaughter, a plea accepted on the grounds of his diminished responsibility. Telling him that he would be held in Broadmoor top security hospital indefinitely, Mr Justice Griffiths Williams said: "You are on any view a very dangerous man."

Ms Nickell, 23, a part-time model, was attacked as she walked on the Common, in south-west London, with her son Alex, then aged two. She was stabbed 49 times and her body was mutilated.

The police investigation focused on Mr Stagg, who lived nearby and had a reputation as something of a loner. In the absence of hard evidence, and with the advice of the forensic psychologist Paul Britton, the police devised a "honeytrap" operation to try and extract a confession from Mr Stagg.

An undercover policewoman, using the name Lizzie James, posed as a potential lover and exchanged a series of letters with Mr Stagg outlining explicit and violent sexual fantasies.

The exchanges formed the basis of the Crown's case when Mr Stagg was charged with the murder. But an Old Bailey judge threw the case out in September 1994 before it reached the jury. Mr Justice Ognall said the undercover entrapment operation was "a blatant attempt to incriminate a suspect by positive and deceptive conduct of the grossest kind”.

Mr Stagg was freed but continued to be vilified in sections of the media. He received £706,000 compensation from a Home Office scheme in August this year.

Napper, 42, was not interviewed as a suspect for the murder until after Mr Stagg's acquittal, despite the fact that he had emerged before the Stagg trial as the key suspect for a series of violent knifepoint rapes and a similar murder of a mother and her child.

Napper denied any involvement in the Wimbledon murder until new DNA techniques produced a match to his profile following extensive tests in 2004. He was charged with Ms Nickell's murder last year following further tests which established the profile was a 1 in 1.4 million match with Napper's DNA.

Today Napper sat in the dock flanked by three staff from Broadmoor. Wearing a blue checked shirt he seemed impassive as details of his crimes and illnesses were outlined.

A few yards away sat Andre Hanscombe, Ms Nickell's former partner. Two rows behind him were Ms Nickell's parents Andrew and Monica.

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