Melanie had fractured skull and jaw

The 25-year-old's bones were found in a bag near the M5 north of Bristol on October 5, 13 years after she went missing following a night out in Bath.

Detective Superintendent Mike Courtiour, who is leading the investigation for Avon and Somerset Police, said a pathologist had been unable to give a definite cause of death.

But he revealed that Miss Hall's body was found covered in black bin bags tied up with a long length of thin blue rope.

The 4mm (0.2in) thick blue rope was found bound around the black bin bags which contained Miss Hall's remains and is still undergoing extensive forensic tests.

Mr Courtiour said the rope, which is commonly used and can be bought from hardware stores, could hold vital clues. He appealed to the public to contact the police if they know anyone who has used a similar type of rope in the past.

He said: "Who had access to similar blue rope 13 years ago in their car, house or garage? There is someone out there who knows who killed Melanie and then bound her body here in this blue rope."

He added: "Showing the blue rope today may just prompt someone to remember.

"It is a widely available rope and we are well aware that people can pick it up from many DIY stores, but someone may know someone who bought it after Melanie's disappearance."

The Home Office pathologist who carried out the post-mortem examination on Miss Hall could not give a definitive cause of death but she had suffered severe blunt trauma to the head, causing a fractured skull, cheekbone and jaw.