The judge, Mr Justice Ognall, said yesterday: 'No one questions in certain cases the assistance of a psychologist of that kind could provide a useful investigative tool - but in this case the police went further.'
A prosecution relying on psychological profiling is without precedent in Britain. In the United States it has secured convictions on 17 occasions, but all have been thrown out on appeal.
Mr Britton, 48, is head of the Trent Regional Forensic Psychology Service in Leicester and works for the police unpaid and in his spare time. In the US, psychological profiling has been used for 40 years, but until a decade ago was almost unknown in Britain.
The High Court was told that Mr Britton had been asked in July 1992 to profile the likely murderer of Rachel Nickell, based on the method of killing, the scene and the choice of victim. Unusually he remained with the case. The court was told that he had devised an undercover operation designed, for the first time in Britain, to secure a conviction by matching the psychological profile of a suspect to that of the likely killer.
A policewoman would encourage the suspect's fantasies, to see where they led. Mr Britton claimed his was such a precise science that a close match would prove guilt beyond reasonable doubt, the court was told. He supervised every outgoing letter and conversation, studied every response and predicted the likely path the suspect's fantasies would follow. At any stage if it diverged he would be able to call off the experiment.
In his view, every new element was introduced by the suspect, with the policewoman merely following and encouraging. And in his view the results were a profile match between suspect and killer.
Mr Britton told a committal hearing in January in which Colin Stagg was sent to trial at the Old Bailey.: 'What I can say to you is that as a consequence of carefully listening to hours of interviews between Mr Stagg and the police and in carefully reviewing his behaviour, his activity during the covert operation, I saw no evidence whatsoever to suggest he would be vulnerable in the sense of Pace, (the Police and Criminal Evidence Act) as I understand the codes of practice.'
Mr Britton insisted the fantasies described by Mr Stagg were not made up to entice someone into bed.
His earlier celebrated cases include telling police that adults were unlikely to have been involved in the murder of two-year-old Jamie Bulger and drawing what turned out to be an accurate picture of what was in the minds of Robert Thompson and Jon Venables before they killed him
He drew up an acurate profile of Michael Sams before he was caught and sentenced for the murder of Julie Dart in Bradford and for the kidnapping of the Birmingham estate agent Stephanie Slater. He was also called in to advise the police while they were seeking the kidnapper of Abbie Humphreys this summer. In 1992, he used his professional standing to help a TV producer infiltrate Albany prison undercover to film serial killer Dennis Nilsen.