Government to bring in mandatory minimum sentence of six months for repeat offenders, which was blocked by Lib Dems in coalition

A mandatory “two strikes and you’re out’’ minimum six-month prison sentence for carrying a knife, which was blocked more than a year ago by the Liberal Democrats, is to be finally introduced next week.

Justice ministers expect more than a thousand extra offenders a year who are repeatedly caught carrying a knife or blade will be jailed under the measure, which will come into effect on Friday 17 July.

The “two strikes” law will mean that adults convicted of being in possession of a blade for a second time will face a prison sentence between a minimum of six months and a maximum of four years. Young offenders aged 16 and 17 will face a minimum four-month detention and training order.

A Ministry of Justice impact assessment shows that around 1,300 adults who already have a knife conviction are caught in possession of a knife each year without receiving an immediate custodial sentence under the current regime. Judges are to be able to exercise their discretion if they think it would be unjust in the circumstances to impose the sentence.

The justice ministry estimates that about 1,000 offenders a year will be jailed as a result of the new move, leading to a need for an extra 350 prison places in already overcrowded jails in England and Wales. A further 110 young offenders are expected to be jailed each year under the measure. Combined, this is estimated to add an extra £10m a year to the costs of the criminal justice system.

Officials acknowledge that the “two strikes” sentence is being introduced at a time when the number of adults being sentenced for possession of a knife or an offensive weapon has fallen by 60% since 2008.

A third, or 3,707, of the 12,125 offenders convicted of possessing a knife in 2014 were given an immediate jail sentence. It is estimated that 1,337 of the 7,944 who were cautioned or given a non-custodial penalty had at least one previous conviction for knife possession.

The official impact assessment adds that the published evidence shows that making sentences more severe only has a minimal impact as a deterrent and that increasing the likelihood of being caught is actually more effective. The officials say that senior police leaders “have offered their view that this policy might have a deterrent effect”.

The mayor of London, Boris Johnson, and the Metropolitan police commissioner, Bernard Hogan-Howe, have been pressing ministers since the election for the new sentence to be introduced after a recent rise in stabbings in London.

Nick Clegg, the former Liberal Democrat leader and deputy prime minister, blocked the measure last June arguing that it was wrong to use a “headline-grabbing solution” which would not necessarily work in practice. He highlighted the example of cases where gang members were forced to carry knives.

“Putting someone like that into prison could push them into the arms of hardened criminals and let the gang leaders off the hook. Instead of prison we need to divert vulnerable people away from gang culture,” he wrote in the Guardian.

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Danny Alexander, then Lib Dem chief secretary to the Treasury, also blocked the move. “The Ministry of Justice’s budget is already under significant strain, and this proposal would certainly not be affordable before 2016-17,” he said in leaked cabinet correspondence in April 2014.

The justice minister, Mike Penning, confirmed that the measure would be introduced on 17 July.

“We are already making sure knife offenders are properly punished and keeping more off the streets for longer, making our communities safer,” he said.

“With this new measure we are sending out the strongest message to offenders: repeatedly take a knife on to our streets and expect to go to prison.”