He added that £245 million a year is spent on detaining 16,000 offenders, who need to be educated whilst in detention, to use the time "sensibly".
Secure colleges are to be considered in a bid to improve the education received by young offenders.
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PM spokesman's comments come following reports that the EU is secretly drawing up plans for greater military cooperation across the EU.
The Labour politicians put their alleged differences aside to jointly highlight how remaining in the EU could help fight climate change.
Late sunshine here and there this evening - but more cloud for southern Scotland, northern England and Northern Ireland again.