Plans for mandatory custodial sentences for 16 and 17-year-olds who threaten others with knives have been unveiled by Justice Secretary Kenneth Clarke.Convicted teenagers would face a four-month detention and training order.The proposals form part of a new sentencing regime for the courts.Also planned is a \"two strikes and you\'re out\" policy which would see a person convicted of a second very serious sexual or violent crime get a mandatory life sentence.Currently in England and Wales, only those convicted of murder face a mandatory life term.The new sentencing plan comes a day after the justice secretary told MPs that judges should have discretion over sentencing.He added that mandatory sentences were not the British way and led to a game in which judges would look for any excuse not to hand down the set terms.Announcing the surprise move, Mr Clarke said: \"We have already announced that we are bringing in an automatic prison sentence for any adults who use a knife to threaten and endanger.\"Clearly any extension of this sentence to children requires very careful consideration.\"However, we need to send out a clear message about the seriousness of juvenile knife crime, so we are proposing to extend a suitable equivalent sentence to 16 and 17-year-olds, but not to younger children.\"Ministry of Justice figures suggest between 200 and 400 teenagers aged 16 and 17 could be jailed every year for using a weapon to threaten others.These latest moves to combat knife crime, which will be included in the Justice Bill when it is debated by MPs in the Commons next week, come after Mr Clarke held talks with Conservative backbencher Nick de Bois.He had demanded the government extend a new mandatory custodial sentence for knife crime to under-18s, and said he was \"very pleased\" with the government\'s decision to change the law as he suggested.Mr de Bois added: \"It sends a very clear signal that in the youth justice courts, for the first time, it\'s beyond doubt you\'ll go to jail if you use a knife in this fashion.\"Further changes to the sentencing regime in courts include:Extending the category of the most serious sexual and violent offences to include child sex offences, terrorism offences and \"causing or allowing the death of a child\" so that the new provisions will apply to them.The Extended Determinate Sentence (EDS) - all dangerous criminals convicted of serious sexual and violent crimes will be imprisoned for at least two-thirds of their sentence, ending the regime which allowed the release of these offenders at the halfway stage.Offenders convicted of the most serious sexual and violent crimes in this category will not be released before the end of their sentence without parole board approval.Extended licence period - criminals who complete an EDS must then serve extended licence periods where they will be closely monitored and returned to prison if necessary.Courts have the power to give up to an extra five years of licence for violent offenders and eight years for sexual offenders on top of their prison sentence.Mr Clarke said the new proposals would \"restore clarity, coherence and common sense to sentencing, rid us of the inconsistent and confusing IPP [Imprisonment for Public Protection] regime and give victims a clearer understanding of how long offenders will actually serve in prison\".Labour\'s shadow justice secretary Sadiq Khan said the plan \"does not address the problem of unreformed offenders who have completed their sentence being released to commit crime and inflict harm on the public\".He added: \"Under this government\'s plans, offenders who are a danger to the public could still be released from prison. They are taking an unnecessary risk with public safety.\"But he welcomed the moves over knife crime, having previously called for mandatory custodial sentences for threatening anyone with a knife to be extended to those under 18.All the new measures will be debated in the House of Commons next week and, if passed, will be added to the Legal Aid, Sentencing and Punishment of Offenders Bill which is currently going through Parliament.