SCOTLAND YARD is to charge a convicted sex killer with the murder of Rachel Nickell, the young mother stabbed to death on Wimbledon Common in 1992.

Police hope the charges can bring to a close one of Britain’s most notorious unsolved murder cases.

In what is seen as a triumph of forensic detective work, murder charges are expected to be laid next week against Robert Napper, a 41-year-old mental patient. Police used the latest scientific techniques to take microscopic DNA particles from Nickell's clothing and “grow” them in the laboratory so that a match could be made.

The tests, part of a 13-year reinvestigation into Nickell’s murder, are said to show a one-in-five-million chance that the DNA did not belong to Napper.

A paranoid schizophrenic, he is being held indefinitely in Broadmoor high security hospital for the killing of another young mother and her daughter.

They died in November 1993, 16 months after Nickell, a 23-year-old part-time model, was stabbed 49 times and sexually assaulted in front of her two-year-old son Alex. She had been walking her dog on Wimbledon Common in August 1992.

Nickell’s murder shocked the nation because of its brutality and the fact that no witnesses could be found, even though it took place in daylight.

Colin Stagg, a loner who lived nearby, was originally charged with her murder but an Old Bailey judge dramatically halted his 1994 trial after condemning an attempt by undercover detectives to catch him in a “honey trap”.

Last year John Reid, the then home secretary, agreed that Stagg should receive compensation. Legal experts say he could get up to £500,000.

Napper is being held indefinitely for the killing of Samantha Bissett and her four-year-old daughter Jazmine in Plumstead, south London.

Bissett, 27, a blonde who bore a strong resemblance to Nickell, was raped and stabbed up to 20 times in the head and neck in the hallway of her home. She was dragged to the living room, where Napper cut her body from the throat to the crotch and removed part of her stomach as a “trophy”. Her daughter was found smothered to death. She had been sexually assaulted.

Known as the Plumstead Ripper, Napper has long been suspected of up to 40 other violent rapes but has never admitted them. At his Old Bailey trial in 1995, he denied two charges of murder but admitted manslaughter on the grounds of diminished responsibility.

Napper was originally ruled out of involvement with Nickell’s murder on the grounds that he did not frequent southwest London. Some officers nevertheless wanted to question him at the time, but were told he was too unwell.

It has since emerged that Napper knew and frequented the area of Nickell’s murder because he had been receiving psychiatric treatment in a clinic next to the common.

Scotland Yard launched a new inquiry into Nickell’s murder after the collapse of Stagg’s trial. The breakthrough came when scientists carried out a microscopic re-examination of Nickell’s underwear.

Scotland Yard and the Crown Prosecution Service called in outside forensic experts after doubts were raised about the original scientific work. Much of the new work was done by LGC Forensics, a British firm that is a world leader in DNA analysis.

Napper has been interviewed at least three times by Scotland Yard’s murder squad and a trial is likely to be scheduled for next year.

Rachel’s son, Alex, is now 18 and living with his father Andre Hanscombe in France. When a passerby first discovered Nickell’s body, Alex was found clinging to it pleading, “Get up, Mummy.”

He was too young to be used as a witness. However, he was able to solve one mystery. A bloody scrap of paper on Nickell’s forehead was at first thought to be the work of the killer. But Alex eventually revealed he had been trying to “make Mummy better” with a plaster.

Additional reporting: Anna Mikhailova

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