A man jailed for life for strangling a teacher to fulfil a sexual fantasy has won an appeal against his conviction.
Jane Longhurst was a special needs teacher and musician in Brighton
Jurors in the trial of Graham Coutts, 36, should have been offered murder and manslaughter verdicts, Law Lords said.
Coutts, of Sussex, was ordered to serve 26 years for murdering Jane Longhurst. His lawyers said manslaughter was an option because of his "lack of intent".
On Wednesday, Ms Longhurst's mother, Liz, of Reading, said she was "aghast" at the Law Lords' ruling.
Miss Longhurst, 31, was found dead on Wiggonholt Common, near Pulborough, West Sussex, on 19 April 2003.
During his trial, Coutts, of Hove, who was obsessed with internet porn, admitted he had been present when the special needs teacher died, but denied murder, saying her death was an accident during consensual sex.
Miss Longhurst was strangled with a pair of tights.
Jurors heard her body was kept in a storage unit for weeks before it was found.
Five Law Lords have now sent Coutts's case back to the Court of Appeal to "invite that court to quash the conviction".
Appeal judges had last year dismissed an appeal by Coutts, ruling the murder conviction was safe, but they reduced his 30-year minimum term to 26 years.
And the Court of Appeal also said the case raised issues of "general public importance" fit for consideration by the House of Lords, which led to the Law Lords' decision on Wednesday.
Coutts's case has now been sent back to the Court of Appeal
Giving the ruling, Lord Bingham said the Court of Appeal may deal with "any application for a retrial which may be made" and he said Coutts would be "remaining in custody meanwhile".
Lord Bingham said the trial judge's failure to leave a manslaughter verdict to the jury "although fully understandable in the circumstances, was a material irregularity".
He said: "While the murder count against the appellant was clearly a strong one, no appellate court can be sure that a jury, fully directed, would not have convicted of manslaughter."
And he said there had been evidence at the trial which would have enabled a jury, if they accepted it, to convict Coutts of manslaughter.
Lord Bingham said public interest was best served "if the trial judge leaves to the jury, subject to any appropriate caution or warning, but irrespective of the wishes of trial counsel, any obvious alternative offence".
After the hearing, Ms Longhurst's mother Liz, of Reading, Berkshire, said: "I'm terribly sorry, I really am so disappointed that this has happened."
She told the BBC: "It was in my mind that it could happen, but I'm really rather aghast."
After her daughter's death, Mrs Longhurst fought a long-running campaign for action to block access to violent internet porn sites.
Trial jurors had heard about Coutts's obsession with strangulation and how he looked at internet sites connected with the fetish.
Mrs Longhurst said the hoped-for legislation, which would apply to websites wherever they were based in the world, would mean her daughter's death had not been "entirely in vain".
Last November, she handed in a petition of more than 50,000 signatures to the House of Commons.