Justice Secretary Kenneth Clarke's plans to automatically jail teenagers convicted of threatening others with a knife for at least two months will be no deterrent against knife crime, campaigners have warned.
Up to 400 teenagers would be jailed every year under the plans to extended mandatory prison sentences to 16- and 17-year-olds, Ministry of Justice estimates showed.
However the four-month minimum detention and training order, the main custodial sentence for under-18s, would see the teenagers spend just two months in custody.
Vicki Helyar-Cardwell, director of the Criminal Justice Alliance which represents more than 60 organisations, added: "Imposing mandatory custodial sentences for teenagers is not an effective response to the problem of knife crime.
"The evidence for the deterrent value of such sentences is non-existent. Short prison sentences are particularly hopeless at addressing offending behaviour, and could end up turning young people into more hardened criminals."
However Mr Clarke, who appeared to be against mandatory sentences earlier this week, said the move was necessary "to send out a clear message about the seriousness of juvenile knife crime".
The proposals increase the use of US-style mandatory sentences, despite Mr Clarke saying they were "a bit of a leap for the British judicial system", "a bit of an innovation" and an American way which goes against the British system.
He told MPs that judges simply find excuses not to apply mandatory sentences when it is clear it would be unjust to do so, saying it was a "game that should not go on between the Parliament and the courts".
The Government has already announced proposals for a mandatory six-month sentence for adults convicted of the same offence.
John Bache, chairman of the Magistrates' Association (MA) Youth Courts Committee added: "Whilst agreeing that removing knives from our streets is of paramount importance, the MA is against mandatory sentences as there are rare but exceptional circumstances when such a custodial sentence may be inappropriate and, whatever the offence, youths lack the maturity of thought of adults and must be treated accordingly."