Appeal judges last week upheld 19-year-old Mitchell's conviction for the killing of his 14-year-old girlfriend in Midlothian in 2003.

But his lawyers revealed yesterday that they will take his case to the Scottish Criminal Cases Review Commission to seek a fresh appeal hearing.

They are determined that the appeal court should examine fresh evidence that it has so far refused to consider relating to a potential alternative suspect for the brutal murder.

Mitchell's legal team expected that the appeal judges, led last week by the Lord President, Lord Hamilton, would uphold the conviction.

One of the team, Nigel Beaumont, said: "He's 19 years old with 16 years of a life sentence still to serve.

"No-one would expect him to give up now when there is still credible evidence that could help his case that has not been fully examined."

The new moves will come as a blow to Jodi's family, who expected last week's appeal hearing would end their post-trial ordeal. They were unavailable for comment yesterday but Jodi's mother, Judith, left court saying the rejection of the appeal was "brilliant" and she was "relieved it was all over".

Mitchell was convicted in January 2005 of a murder that shocked the world. The killer and his victim, both from Dalkeith, were just 14 at the time of June 30, 2003, when Jodi was killed in a ferocious knife attack. Her virtually naked body was found in a wooded area alongside a path that ran between their homes.

Police suspicions against Mitchell were first aroused by the fact that he discovered her body during a search conducted by her friends and relatives. Others who were present later told detectives that the body was not visible from the path, yet Mitchell appeared to have led them straight to it.

But Mitchell's legal team believe this is in part why the fresh evidence is crucial.

A source close to the team said: "The police said publicly that from the very start they kept an open mind, that they spent months examining every witness statement, every piece of information, and did not make their move until they had eliminated every other possibility.

"Yet many months after the trial and conviction, they receive a witness statement from someone who had told the police at the time of a potential suspect.

"Not only did the Crown not pass that information to the defence, as they should have done under disclosure rules, they did not consider the alternative suspect seriously because their mind was made up."

The alternative suspect was named in appeal papers submitted by Mitchell's legal team as Mark Kane. At the time of the murder, he was on a residential course at Newbattle Abbey College for people recovering from addiction problems.

But the appeal court was told that Kane had been cleared by DNA tests.

Advocate depute John Beckett QC, for the Crown, also told the judges that Kane had been questioned by police and had told them: "I know nothing else about the murder of Jodi Jones."

It was alleged by a friend of Kane's that he often took alcohol and cannabis into the woods, close to where Jodi was murdered, to avoid detection by course supervisors.

Like Mitchell, he was a devotee of the rock band Nirvana and the singer Marilyn Manson, whose pictures of the infamous Black Dahlia murder victim Elizabeth Short were believed to have influenced the killer when mutilating Jodi's body.

Kane also had a fascination with websites that displayed graphic images of violence and would show his favourite scenes, such as people being impaled on spikes, to his fellow-students.

He was suspected by college authorities of being responsible for defacing doors and walls with a heavy knife or chisel with the words 'kill' and 'die' and the Nazi swastika. Three weeks before Jodi's murder, he penned an essay entitled 'Killing a female in the woods'.

On the day after the murder, he had scratches to his face and arms, particularly under the eye, and claimed he could not remember how he got them, but when pressed later gave at least three different accounts.

The source said: "Here was someone who had an interest in graphic violence against women. He wasn't in Inverness or Aberdeen. He was right on the doorstep at the time of the murder.

"The standard for admitting fresh evidence at the appeal stage is that it could not have been made available at the time and is significant enough that it could have had a material impact on the jury's verdict.

"The defence could not make available what it had not been told. As for the second part, I think the jury would have been interested to know there was an alternative suspect to a 14-year-old boy."