Minimum term for Aaron Campbell is reduced from 27 years to 24 on account of his age at the time of the killing

The teenager convicted of the rape and murder of the six-year-old schoolgirl Alesha MacPhail has had his 27-year minimum sentence reduced.

Lawyers for Aaron Campbell, who was 16 when he was jailed for killing the child on the Isle of Bute in Scotland, had argued that his sentencing was “excessive and amounted to a miscarriage of justice”.

In a judgment published on Tuesday, three judges ruled the sentence should be reduced to 24 years on account of the killer’s age at the time of the murder.

The ruling sparked criticism in Scotland, with the Scottish Conservative party calling it a “disgraceful outcome” that would cause massive upset to Alesha’s family.

“This is one of the most vile criminals in Scotland, and he should not be enjoying having time shaved off his sentence,” said the Scottish Conservative shadow justice secretary, Liam Kerr.

The defence QC Brian McConnachie, who represented Campbell during his trial in February, had argued in court in Edinburgh last month that the killing in July last year was “an appalling and heinous crime”.

But he said the trial judge, Lord Matthews, had attached too great an importance to two sentencing reports that were unduly pessimistic about Campbell’s capacity for rehabilitation, and thus imposed an excessive “punishment part” of 27 years to his life sentence.

In their judgment, the appeal judges concluded that the extent to which Campbell was likely to present a future risk, the appalling nature of the crime and the bleak prospects for change “led the trial judge to make inadequate allowance for the mitigatory effect of youth”.

Facebook Twitter Pinterest A picture of Alesha, soft toys and flowers were left on Bute in remembrance of the six-year-old. Photograph: Murdo Macleod/The Guardian

Campbell’s dysfunctional home background, absence of boundaries and parenting problems, “including a degree of physical and emotional abuse”, were noted in the ruling from Lord Drummond Young, Lord Menzies and Lady Dorrian.

These are all factors that may have adversely impacted on the development of his character, the judges found, “but which may not entirely dictate development of his character in the future”.

“We have concluded that a punishment part of 24 years would be appropriate to reflect the appellant’s youth,” they added.

“As with all punishment parts, this is not an indication of the date when the appellant will be released. It specifies rather the period which must pass before the appellant may even apply for parole.”

They added: “As the trial judge had observed … ‘whether [Campbell] will ever be released will be for others to determine but as matters stand a lot of work will have to be done to change [him] before that could be considered – it may even be impossible.”

Alesha, from Airdrie in North Lanarkshire, had arrived on the Isle of Bute a few days earlier for a three-week summer break. After she was reported missing there was a frantic search before a member of the public found Alesha’s body.

During the two-week trial, the jury heard that Campbell had taken Alesha from her bed in a seafront flat where she was staying with her father and grandparents, using a knife to silence her, and had carried her to the nearby grounds of a disused hotel where he had raped and smothered her.