The life sentence handed down to the 16-year-old schoolboy who murdered teacher Ann Maguire has been condemned as too harsh by youth justice campaigners.

Will Cornick will serve at least 20 years for killing Mrs Maguire, 61, who was stabbed seven times from behind as she taught a ­Spanish class at Corpus Christi Catholic College in Leeds in April.

Mr Justice Coulson warned Cornick, who was 15 at the time, that he might never be released from prison as he passed sentence at Leeds Crown Court.

But Penelope Gibbs, who chairs the Standing Committee for Youth Justice umbrella group of charities and campaign groups, said the sentence was too long.

She said: "We are out of line with the whole of western Europe. There are no other countries within western Europe which would give children a life sentence - and this boy is seen as a child under the UN Convention on the Rights of the Child and is in the youth justice system."

Ms Gibbs accepted that the sentence had to serve as a punishment, but said there was "no evidence" for the 20-year minimum tariff.

"Punishment is also incredibly important, ­particularly for the victims and families, but how many years do we need for punishment?" she said

"We have given him a sentence which is more than his own lifetime."