The savage murder of Rachel Nickell on Wimbledon Common in July 1992 remains unsolved.

Colin Stagg, the man who was accused of her murder, could reasonably have expected that his 15 minutes of celebrity, as predicted by Andy Warhol, would have ended with his acquittal in September 1994. He could have expected to be free and anonymous once more after the trial judge ruled that much of the evidence brought against him was inadmissible before going on to roundly criticise the Crown Prosecution Service (CPS).

But on Sunday 20 October the Mail on Sunday sensationally printed evidence relating to the case, including statements given by an undercover policewoman known to Stagg as “Lizzie James”, that were not heard by the jury. Stagg was back in the media spotlight.

The newspaper claimed that he gave the policewoman details of how Rachel's body was placed on the ground – information that was not disclosed by detectives to the public. To the trial judge the evidence was “thoroughly reprehensible” and ruled against its use by the prosecution. However, the evidence was presented to magistrates during the 11-day committal of Stagg but was not reported because of the usual legal restrictions.

Magistrates also heard evidence from neighbour Susan Gayle, who claimed to have seen Stagg on Wimbledon Common at 9.25am on the day of the murder, contrary to what Stagg told the police. He claimed that he was home by 9.15am, having left the common after walking his dog that morning.

The revelation of the evidence caused a media uproar and was, in the opinion of Lord Denning, former Master of the Rolls, in contempt of Court. But the CPS confirmed: “Mr Stagg was acquitted of the murder of Rachel Nickell and cannot be tried again for the same offence.” Therefore there is no trial to be prejudiced save in the event that another defendant is subsequently charged and appears before the Court. Scotland Yard maintains that its investigations are continuing but there is no indication that an arrest is imminent.

Despite his acquittal, the stigma of the police investigation has brought Stagg into “public scandal, odium and contempt”. His blue Audi car was allegedly stolen from outside his home on the day the Mail on Sunday article appeared and, he maintains, he has been harassed by the press since his acquittal.

As one of his lawyers, James Sturman, points out: “It was not his fault that the Crown's case collapsed. He would have in excess of 55 witnesses to demonstrate his innocence.” William Clegg, the lawyer who defended Stagg, wrote to The Times and accused the media of pursuing a campaign against his client. He believes that the material published in the Mail on Sunday demanded correction because the case against his client was based upon a psychological profile of the killer which was alleged to match Stagg.

He said: “During a five- month police undercover operation designed to obtain a genuine confession all that was obtained from Colin Stagg was a confession to a crime that never took place and a number of details about the murder of Rachel Nickell that could have been proved false.”

The reason the case was dropped, according to Mr Clegg, was because the Crown Prosecution Service offered no evidence as they could not, and did not, prove Stagg guilty of the killing. Had he been convicted, Stagg would have been unable to sue for libel because of Section 13(1) of the Civil Evidence Act 1968, which provides that the fact that a libel plaintiff stands convicted of an offence is conclusive evidence that he committed the crime.

This was enacted after a convicted criminal sued for libel and the publisher was obliged, in effect, to reconvict the plaintiff in front of a different jury. But as Stagg was not convicted he would be entitled to sue for defamation, although, of course, the costs in bringing such an action would be substantial.

If the newspaper was unable to prove his guilt (and if the Mail on Sunday was unable to show that taken as a whole the article did not mean that he actually was the murderer) huge damages would be likely to be awarded.

Although in recent celebrity libel cases Court of Appeal judges have been keen to limit the perceived excesses of juries (for example, Elton John and Mirror Group Newspapers,

Esther Rantzen's action against the People), substantial six-figure damages would still be appropriate if a jury was convinced by Stagg's version of the events and not that of the newspaper.

A similar situation occurred when retired police officer Gordon Anglesea was defamed by newspapers including The Observer and The Independent on Sunday. The defendant publishers pleaded justification and called witnesses to support their assertions that the plaintiff had been guilty of sexually abusing children at a children's home in North Wales. The jury, however, found in favour of the plaintiff and probably would have awarded him vast damages to compensate for the effects of the articles, a television programme, and an item in Private Eye, all exacerbated by the attempt to prove the truth of the allegations during a long and hard fought trial.

But, no doubt to avoid the risk of an appeal on the quantum of damages, the parties agreed the level of compensation before the jury had reached its decision, which amounted, including costs, to several hundred thousand pounds.

To discourage the press from making untrue allegations of criminality against individuals the burden of proof in libel is placed upon publishers, and where serious crime is alleged it is in effect as high as that faced by the Crown in prosecutions. The jury has to be satisfied rather than the plaintiff having to prove his case in the usual civil action manner.

It is established law that the fact that a plaintiff in a libel action has been acquitted of a criminal offence does not preclude a newspaper from setting up a plea of justification but, as set out in Gatley on Libel and Slander, a publisher would “find it very difficult to establish his plea, and failure to do so would almost certainly aggravate the damages”.

No doubt the Mail on Sunday gave the story careful consideration before publishing. Should Stagg decide to sue its publishers (and it was reported in the press that he was consulting with his lawyers), he would need to satisfy himself that the article was not protected by the privilege given to fair and accurate reports of court proceedings and that the article, taken as a whole and not just its headlines, was defamatory.

But the Mail on Sunday was hard-hitting and asked: “Was Colin Stagg right, mistaken or lying about the time he took the walk on the common? Has he given a time so far removed from the real time of the murder because he knew exactly when the killing occurred?”

Although one can imagine the state of mind of Stagg and his wife when they read the newspapers on Sunday and subsequently during the week, it can be argued that it is better that such allegations should be published and publicly scrutinised than suppressed by fears of contempt of court or libel writs.

The mystery surrounding the murder of Rachel Nickell remains a matter of continuing public interest and concern. If the Mail on Sunday is wrong, a new level of libel damages may be on the way.