A man who killed a teacher and then kept her body for five weeks was facing a retrial yesterday after Law Lords directed that his murder conviction should be quashed.

The mother of victim Jane Longhurst, a special needs teacher and talented musician, said that she was "aghast" that the conviction of Graham Coutts, 36, was to be overturned.

Liz Longhurst has been campaigning for a ban on violent internet porn sites since it emerged at Coutts's trial that he subscribed to websites specialising in rape and female asphyxiation.

Coutts strangled Miss Longhurst, 31 - the best friend of his pregnant lover - in his flat in Hove, East Sussex, in 2003.

He then kept her body for five weeks in a box in a storage depot, visiting it frequently, before driving it into the countryside and setting fire to it.

During his trial at Lewes Crown Court in 2004, Coutts admitted he had been there when she died, but denied murder, saying her death was an accident during consensual, asphyxial sex.

His appeal against his murder conviction was rejected by the Court of Appeal last year, although the 30-year minimum sentence imposed by the trial judge was reduced to 26 years.

Yesterday, however, five Law Lords sent the case back to the Appeal Court and "invited that court to quash the conviction" because the trial jury had not been offered the opportunity to consider a verdict of manslaughter. Coutts's lawyers had argued that manslaughter was an option because of his "lack of intent".

Announcing the decision, Lord Bingham said that the Court of Appeal may deal with "any application for a retrial which may be made" and he said that Coutts should remain in custody in the interim. The Crown Prosecution announced immediately that it would seek such a re-trial.

Lord Bingham said the original trial judge's failure to offer a manslaughter verdict to the jury was a material irregularity, "although fully understandable in the circumstances".

After the ruling, Mrs Longhurst, 74, a retired teacher from Reading, said: "I really am so disappointed that this has happened. It was in my mind that it could happen but I'm really rather aghast."

At his trial, the jury took just over nine hours to reach a unanimous verdict that Coutts, a part-time musician, was guilty of murdering Miss Longhurst after luring her to the flat he shared with his girlfriend, Lisa Stephens, and strangling her with a pair of her tights.