Justin Hopkins, 19, was 15 when he tried to strangle the 14-year-old while the pair were alone at Warwick Racecourse in January 2004.

His original sentence of nine years in a young offenders institution was reduced to six years, and Hopkins could have been released after only half this time, but parole board officials decided that he was not suitable.

Now High Court judge Mr Justice Stadlen has ruled the board's decision not to allow Hopkins to put his case was "unlawful".

Mr Justice Stadlen said some prisoners' conduct and attitude counted "overwhelmingly" against their chances of release, but Hopkins was not one of these.

On January 20, 2004, the defendant had been with a group of other people in Warwick when he and his victim both decided to take a shortcut across the racecourse.

When they were at an isolated part of the racecourse Hopkins attacked the girl and then decided to strangle her with his belt.

Hopkins later admitted to assault occasioning actual bodily harm, but denied attempted murder. A jury convicted him of the more serious offence.

Although she suffered relatively minor injuries, the trial judge had no doubt the victim had "suffered very grave distress" and was "still bearing the mental scars".

At the High Court, Mr Justice Stadlen said that factors including the "gravity" of the offence and Hopkins' submissive personality, which led to the risk of sudden outbursts of violence, weighed against his chance of release.

But he added that two probation officers recommended release and considered that the risk could be managed satisfactorily in the community.

He said: "I am not satisfied that it was fair to deprive Mr Hopkins of the opportunity of persuading the panel that the views of the probation officers were correct."